The third option is the one that's the most time consuming but also has its benefits. It's like having a successful restaurant and buying another restaurant and operating them simultaneously. They're not the exact same restaurant, but both are popular in their own right and make you money. The same goes for Option #3. You could update the content on the old domain and sell the same products that you're selling on your current site. If you can get both sites to rank alongside each other in the SERPs, you're increasing your conversion chances and sales potential.
Selling on auction sites is another great way to find buyers. Ebay.com is perhaps, the most popular place to sell domain names. Afternic.com is another site that allows sellers to find good buyers. If you have premium domain names, become a member at GreatDomains.com and list your domain name there. The site is a reputed marketplace and brokers deal with thousands of dollars trading domain names. You can also sell domain names through Sedo.com, a site which has its own selling program. Sedo.com has the largest marketplace in the world, and boasts of having a list of the most expensive domain names in the market. It also has a safe and secure escrow service for domain name buyers and sellers.
Also, Sedo.com, makes millions a week selling domains,even in the past week[3]! If you want to sell here, I would probably use a paid category or featured listings, one of my category listings, got 44 views in just 1 week but still waiting for a sale this month, hopefully. I would not really on this as a business, unless you add web development to it, and also, be sure to check that you are making a good profit, say within 6 months!
HIM: "Mainly small-medium sized local businesses looking for a way to increase their ranking. Most already have a branded domain in place, and have heard that using a keyword domain can help. Or they have seen a competitor ranking using that method. The problem I face is that they don't have a lot of money to spend, so I get lowballed on the asking price. There have been a few that make a ton of profit vs what I purchased it for, but that boils down to luck: what is for sale, when I find it, things like that."
There's a lot of guesswork that goes into domain name investing. It's not enough to purchase domain names that are currently popular. Just like a stock, coming too late to the game won't be profitable. You have to think of the future, instead. What domain names might be profitable in the future? Is there a new product or service that may become a huge hit? Where is the future headed?

A suspiciously high number of backlinks from some related niche websites that aren’t too great in quality. In my opinion, this could be an indicator that the domain name was using a Private Blog Network (PBN) service or their own PBN to boost their authority…this blackhat tactic can work, but it places your domain name at risk and the PBN owner could pull backlinks at any time.
Each of the third parties we integrate with charge for getting metrics for a domain. DomCop pays for their API to get this data for you and therefore you do not need to purchase additional accounts to get these metrics. One DomCop account will do it all. As new players enter the market, we integrate with them so that you always have the best and most comprehensive set of metrics to help you decide which domain to buy.
However, I'd argue that most people in the business really struggle as they don't understand what makes a good domain name and what's a bad domain name. While I do not work in this field full-time, I can say that I can make a living of it, but it's probably because I don't ask for absurd prices (e.g. randomword247.com domain name for $8000 is just unrealistic and won't happen) and therefore can sell domains easier. 
While numbers like those above are impressive, most domains sell for significantly less extravagant prices and are generally in the two or three-digit range. Not long ago, those who were able to secure general terms (like icecream.com or pizza.com) not protected by trademark rights often found themselves sitting on virtual gold mines. The glory days of this boom have long since passed, and those looking for a profit in today’s market needto have a keen sense for coming trends.
Here's an old post on search engine roundtable that claims google's policy is to discount previous backlink juice when a domain changes ownership.  I'm not convinced whether this is actually true or something Google says to discourage excessive domain buying / 301 redirecting for SEO benefit.  The comments above seem to give varying opinions on this matter. Would be great to get to the bottom of that one!
Be careful! When searching for domains, it might be tempting to, for instance, buy mcdonalds.net (if it were available) and then try to sell it to McDonald's. The logic here would be that they own the .com and wouldn’t want anyone to own the .net and use their name, right? Also, it’s a big name so there’s gotta be a biiiiiiig payday comin’, RIGHT? Wrong, and wrong.
Listen to expert domainers on domainsherpa.com. Read anything by Michael Berkens, Frank Schilling, Eliot Silver, Morgan Linton, and Ron Jackson. Watch YouTube videos on domain investing. Read blogs like The Industry News Magazine at DNJournal.com. Search #domains on Twitter. Go to industry conferences. Understand the Google Keyword Planner tool inside and out.
Hi I totally agree with you Domaineer42. My partner and I are web developers and are seeing a lot of ‘hype’ selling and not enough ‘real value’ selling based on actual search engine criteria. Recently brokers have knocked back selling domains with great potential value based on some pie-in-the-sky logic, defying even Estibot values we’re told to use. The domains were not our best but with four figure value, according to Estibot, I thought it was a great start as a newbie to domaining. In an industry which should flourish, some of these domain ‘fashionistas’ are making rods for their own backs by devaluing domains that have REAL potential value based on customer search. I don’t understand what is going on, but I have decided to opt out for awhile. We have quite a few domains in our repository, with data backed search criteria, trending upwards. Will bring them out of the closet when the industry is ready to get real.

Yes, buying existing domain names can be profitable too. Existing domains with a potential for profit can be found using Sedo. Make sure you pick domain names that have good traffic, backlinks and Google PageRank under 17 characters. Also, steer clear of existing domain names containing special characters or numbers because they are not likely to be sold.

Thanks for this post. I do good with Adsense and wanted to branch out so I bought 5 domains from Namejet after reading this thread. After the 5 auctions ended I spent $500 total. I contacted 30 people about these domains and after a few bites I was able to sell one of them for $500 so I got my money back and now what I sell the other four for is all profit. The best part about this is that it is not very time consuming. I spent maybe an hour on Namejet. I spent maybe two hours sending out emails. Thanks again!
So the keyword planner came up with 332 keywords related to ripple, most of which can be used to inspire a domain registration related to ripple. Let’s go ahead and get some more ideas. We already know that ripple is another cryptocurrency. Let’s head to NameBio and explore crypto-related domain sales. We’ll use bitcoin for this experiment. I’ll type in “bitcoin” in NameBio search, select “show all” and then sort by price by clicking on the price column. Here’s what it looks like:
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