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.
The domain name industry is quite similar to the real estate industry in a lot of aspects. There are end users, brokers, consultants and domain flippers or “domainers”. Domain flipping works similarly to buying a house, renovating it (or even sometimes just sitting on it) and then selling it again at a higher price point. The gist of it is: you’re purchasing a domain name and betting it’s worth (or will be worth) more than you paid for it. If you’re right, you get a nice paycheck and move on. Those who make a living out of this just rinse, repeat, and scale.