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.
Great article on what to look for when buying expired domains! I’ve looked at the domain and page authority of expired domains but haven’t checked if they are blocked by Google like you’ve listed. I haven’t pulled the trigger yet to purchase one (probably good that I didn’t since I didn’t do all my homework) but this will make it easier to finally go. Thanks for the great info!
The term flipping implies a sale that is done in a flip, or in a quick and sudden manner. You cannot be considered a domain flipper if all you do is just list your domain names and wait for years for them to sell. At its core, domain flipping is about spotting the right opportunities at the right time, involving strategic buying and selling of websites for profit.
Check backlink profile: If you’re not familiar with what backlinks are, they’re basically any public website that “links” to the domain in question. Since there are numerous shady tactics in the SEO world, expired domains’ backlink profiles should be examined carefully. Check if too many links are coming from the same domain or if links have weird anchor texts/use foreign languages. You can use a tool such as ahrefs for these purposes.

Something wise my father once told me "Something is only worth how much someone is willing to pay for it." This small seemingly unimportant statement has guided me in many selling and purchasing decisions in my life. Sometimes, it makes the reality all too apparent. So, is buying a domain with the intention of selling it a good idea? Let's break down the details, and talk to some people that actively pursue this method. Yeah, we know a guy.

As people who normally use internet only to check emails, shop, tweet, upload pictures, update facebook status or conduct some business, a large majority of us remain oblivious to the domain name and domain flipping industry. So, when we do decide to venture into it for some reason, it is only natural for us to carry some commonly known myths associated with it. It can also be easier than building a website from a scratch. Although with easy to follow guides relating to how to create a blog – many are choosing to go down this route and add value to their domain names rather than selling undeveloped.
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.
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.