If you buy keyword specific domains, you're really buying the type in traffic.  I use the URL builder and redirect through that URL so you can see how much traffic your getting from the keyword domain.  There seems to be no rythme or reason to what keyword domains deliver traffic and what don't.  By tracking traffic with the Google URL builder you get a feel for what names are giving you traffic and which are not. ie. the plural, the singular, two words, three words, the possessive, etc.
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.
If you buy a domain from a 3rd party, the backlinks are valuable.  I always wait a few months before redirecting, so just set up a quick minisite until two page rank updates have passed and then go ahead and 301 redirect.  However, it seems that you'll get the link juice from the back links, the anchor text is ignored by Google.   Buying a domain at Godaddy TDName expired auctions, seem to not count as a dropped domain if you immediately set up a mini-site.

There’s an important distinction to make here between domain flipping and website flipping. The latter mainly refers to buying and selling full websites. By full websites, I mean websites that actually have content and more often than not have traffic and revenue. When buying and selling websites, the domain name matters less as the main value there is the content, traffic, revenue, history/reputation, sustainability and growth opportunities. The same can’t be said for domain names where you’re just selling “the name” and hence it’s all that really matters.