Unfortunately, you probably won't like my tips. You need to look for quality domains at a good price and be willing to hold on to them to get your money back or know when you've invested wrong and get rid of them. Quality is in the eye of the holder, and believe me, I've looked at plenty of domain portfolios that the owner just knew were million dollar domains but in my opinion, they'd be hard pressed to get anyone to take them off their hands for just the registration costs. You have to figure that most of the best .com domains were registered in the 90's, and those are the ones you typically see the 6-7+ figure sale prices on. I'm not saying there isn't money to be made because there is. But it does take time, research and money, and maybe in some cases a little bit of luck. If you're thinking of getting into the industry, I recommend digging into the resources provided by Michael Cyger on http://domainsherpa.com and from there, moving to some of the other big blogs such as TheDomains.com or
I've seen some people comment about losing the link value of domains that have expired and picked up, but what about non-expired domains?  Let's say a competitor is going out of business and they still have a year or two left, and we buy their domain and site.  Is there a risk changing registrant info and registrars, even if I keep their site up and mostly the same as before?  I was under the impression that I'd want to keep it under their name so as not to hit the uber-reset button on the domain's inbound link value.
There is a misconception that good ‘.COM’ domain names are hard to find and so they are more expensive to buy. But once again, this statement holds no water. You can easily check out the ‘.COM’ domain names that are on sale at highly affordable prices. However, the only downside to flipping ‘.COM’ domain names is that you will have to discover them at different places like Flippa.com or private seller portfolios.
Let’s play this out with a real example. Say you’re familiar with the real estate market in Tempe, Ariz., and you have the opportunity to purchase tempeapartments.com for $200. This might be a good deal. Tempe has a lot of rental property; it’s  a competitive market; and there’s ample turnover in the apartment space because the city is home to a major university. Ask yourself:
There are many risks that would-be domain investors should carefully consider before buying and selling. The three largest risks are liquidity, subjectivity and legality, but there are also many other ranging from misleading appraisals to faulty escrow payments. Would-be buyers should carefully consider these risks before investing in domain names.
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. 
Many people think of domain buying and selling to be one of the easiest methods of making quick money. Perhaps, you may have heard of someone buying a domain name for under $3000 and then selling it for $25,000. A quick $22,000 profit! Such stories can be very motivating! And that sort of money does exchange hands (Go through 10 of the biggest domain name sales of all time!). But these deals don’t come as easy as they seem! Ask anyone who flips domains for a living and he/she’ll tell you the amount of hard work and skill that goes into it.

Whether you can use an existing backlink profile for a new web project under the available domain depends on whether the link target of incoming links can be reconstructed. In order to analyze which content was provided under a domain in the past, you can find out on the Internet Archive Backlinks have no value if they lead to nowhere and this can even have a negative impact. They are possibly redirected to similar subpages or homepages using 301 redirect. In this case the most part of the link’s power is lost.

Hi guys, just a thought, you do not need to build traffic to the domain yourself, you can always buy expired domains, best to subscribe to a paid service like dropping.com (so you are the first to know) or a free one, expireddomains.net and in this way, you can get old domains that have traffic. You could use these domains for parking, getting advertising income or even to sell. I have noticed, that domains that were not hot in the past, can still be hot now, just use Google Trends to type in the keyword, and you will see its future projections. 
Building a solid domain portfolio is not a piece of cake; it may take you several months or years to achieve this goal but it will definitely be worth it in the end. When you are in the fishing phase, look at the potential of the domain name. Another common strategy used by seasoned domain flippers is to get hold of some high profile domain names and sit on them until they soar in terms of value.
That said, my recommended budget for beginners would be $500+. Using this budget you could buy a bunch of high-potential $10 domain names, expired/dropped domain names or a mix of both. It’s very important to invest only what you can afford to lose and treat this as a side hustle till you get the momentum going. As you gain experience, industry expertise and some sales under your belt, you can then consider slowly growing your business into a full-time gig.