a) The general consensus is that non-dotcoms have an inherently lower resale value but I'm not aware of any technical reasons relating to link popularity.
b) Because Google is the most important search engine on the Internet, my personal view is that you're not talking about link popularity generally, but rather Google Link popularity, or to give it its true name, PageRank. A website that has a high Google PageRank will be resellable, especially if its PageRank were 8 or above (I've seen figures of $5,000 mentioned). In the Google world, residual traffic is just a side benefit, albeit a very nice one.
Tied in to this is whether the domain in question has a Yahoo and/or DMOZ listing. The value of each of these is to enhance your Google PageRank, and many people like the idea of saving $299 by buying an expired address that is listed in Yahoo. Google have gone on record though saying that they do not approve of this manipulation of PageRank, and are likely to manually penalise your site if they catch you playing this game. One of the reasons an expired address is dodgy, is that your new site content is likely to be off topic, especially if you are replacing a previously content-rich website with a generic spammy search engine.