BuyDomains has posted here in the past looking to buy names of resellers, but they only do so on very good names at the kind of rates you could probably get from someone else for them fairly easily, from my limited experience with them.
Rumor has it that they get % off deals with drop places for being frequent customers, so if they bid 5K they may not be paying that at all. That'd explain why they don't pay normal sellers amounts like that. Plus, with a list of names like they have, they don't really need to try to track down individuals.