With over 25 million .com domains registered with Google alone, this top-level domain is by far the most popular choice worldwide. According to the domain marketplace Sedo, the average sales price for a .com domain name during the second quarter of 2015 was 4,701 dollars. The most expensive publicly traded domain names have been known to fetch eight-figures. But don’t quit your day job just yet: such sales are the exception rather than the rule.
METHOD 1: Find a domain name with huge potential and sell it as is – This option is tricky since many good domain names have already been bought, so your chance of being the next Mike Mann is highly unlikely. Understand that unless you have a single, generic word as domain name like these world’s highest-selling domains, it could take months or years to for your domains to sell.
Just because a domain has expired, doesn’t mean it has to be kicked to the curb immediately. It is worth considering turning the internet address into a parked domain for a certain period of time in order to generate advertising revenue. This allows domain owners to bide their time and come up with a new web project for the domain or to find a suitable buyer.
What about option #4 - Redirect your existing domain to the old domain? I bought an old domain that is 100% relevant to my current domain but currently has with very little content. It did have more content fours years ago. The old domain is 13 years old, pr=3, while my current is 7 months old pr=1 and a decent amount of content. The old domain I purchased was not expired though and I do not know if this makes a difference. What are the pros and cons of option #4? Am I correct to think that option #1 would result in no benefit from the old domain's age value and if so why is it not listed as a con, a MAJOR one. Its hard to believe that a 301 using option 1 would give my existing domain 13 years credit but I'll take it if it does.

Content setup: If your domain has potential to attract good search engine traffic, you might want to set up some content to help facilitate that. If this helps your domain do better in the search engine rankings then it’s a big win regardless of whether you choose to flip the domain only or flip the domain with the content as a “website”. The SEO value will be helpful either way.

Simply put, domain flipping is the process of acquiring a domain name with a motive of selling it at a higher cost to someone else. This is very similar to flipping houses or cars, the only difference being that you can’t do much to increase the value of the asset by ‘fixing it up’ and then turning it over for a quick buck. Acquiring a valuable domain name before it is tagged with a premium price is the key to a successful domain flip. Raymond Hackney, a domain investor and consultant, successfully flipped a .website domain (his first on a new domain extension) for a decent profit. Read all about it here.
I have some pot related ones I've purchased recently that deal with the manufacturing of hemp like hempmfr.com, hempmoulding.com, hemp-textiles.com, hempecofuel.com etc.  Some of the older domains I've been sitting on are ones like internetpot.com, califunk.com, cafunk.com, caedibles.com, 1tokewonder.com, govherb.com, grasspeddlers.com, pot-megastore.com, usgov420.com etc.  Another interesting recent on I've purchased is Dogparkdates.com and dogpark-dating.com. Do any of these seem valuable in your eyes?
The easiest and least time consuming option is to 301 redirect the old domain to your existing site. This tactic obviously works best if both sites are in the same sector and are targeting the same keywords; otherwise, if you have a pet supply site and you buy an old Texas Hold 'Em poker site, a redirect probably might raise some eyebrows among the search engines. If, however, your site is brandnamepets.com and you buy onlinepetsupply.com and 301 redirect the domain over, you're inheriting a lot of topical and appropriate links.

In order to maintain ownership of a domain, you must renew it yearly. If you don’t, it “expires”. An expired domain goes through several stages before being released again for general registration. The process differs slightly from one registrar to another but it’s more or less the same. GoDaddy explains what happens to domains that expire with them here.

Since the Buyer pays Escrow.com and not the Seller, Escrow.com can withhold payment until we're satisfied the domain name has been transferred by the Seller. One of the ways Escrow.com does this is by checking the WHOIS database of the appropriate Registrar to make certain it properly reflects the new Buyer's name as the domain name Registrant. Once this has been verified, Escrow.com releases payment to the Seller.
Using the keyword search feature you’d also be able to familiarize yourself with how much domains typically cost on average in a certain industry or niche. Even better, they have an addictive little app called “The Domain Game” available for iOS and Android. They basically give you a domain and you try to guess whether it sold for three, four, five or six figures. You get points for each correct answer and lose points for incorrect ones. You do this for a while and you start to see patterns. Your brain starts connecting the dots and you get better it. Now you’re ready to go out there and scout some domains!