If you buy a dropped domain from Snap or Namejet, the backlinks seem to be worthless for SEO.  They are valuable for traffic if it's targeted to your site.  Go ahead and 301 redirect into your site because it's the traffic from the back links that is worth something.  I use the Google URL builder to redirect these names so you can see the domain the traffic is coming from.

Length: This is pretty self-explanatory. Generally the shorter the domain name, the more valuable it is. This difference in value decreases exponentially as the name gets longer, though. For instance, the difference in price between 3 and 4 letter domains are significant, while the price difference between a 7 and 8 letter domain would usually be much less significant.

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!
Use the advanced search option to quickly hone in on the type of names you are interested in. You can narrow the results by price, top-level domain (i.e, .com, .net, .org, .club, etc.), keyword, and many more filters. Using this feature will help you quickly sort through the millions of domains on the aftermarket and find the domain names that best fit your end goals.
The less tech-savvy population on the internet is not fully aware of what the term ‘domain flipping’ means. If you are one of them, then this article is definitely a must-read for you. In this article, we will delve into the details of what ‘domain flipping’ refers to, how it is done, and whether it is a profitable part-time business opportunity or not.
I don't necessarily agree with some that dropped domains don't have value for SEO purposes but either way one thing you can do is to check out our tool for finding auction domain names (the tool has a database of domains available buy from GoDaddy and SEDO and also give additional SEO criteria to look up for the domains like backlinks, Compete data, CPC, Google stats, Alexa stats, etc. and there is also an option to schedule auction domain alerts so that you can be notified via email if a domain name meeting criteria you specify gets put up for sale) or you could use our tool fo domains dropping soon that shows all domains dropping in the next 5 days that way you can be ready to snap them up right away.

The first thing you need to do is signup at Namejet in order to get your account (signup is free).  You can then use their search functionality to sort through all the upcoming domains for auction.  Place bids for $69 on the ones you want to go to auction for.  If you bid on 100 different domains for $69 each, and there is 50 bidders for each auction you need to understand you aren’t going to owe any money, you are not committing $6,900 to Namejet.  By entering that $69 you are getting an entry into the live bidding when the 30 days expire.  Only if you are the ONLY bidder on a domain would you automatically win the domain and owe $69 to Namejet.

All very relevant points for anyone wanting to sell a domain name. Sometimes it is the basic points that we need to be reminded of in a transaction like this. What is the potential of selling a .com domain that has a relevant search term to a particular industry, but hasn’t been used on a website yet? So it is a good search term but no current traffic.
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!