Jamesosix's answer, although correct is hardly helpful to the OP who I think was looking for a bit more detail. Which I shall endeavour to provide;
There is such a thing as reseller pricing. Resellers will always pay less to maximise profit and because there's a chance their purchase might not even sell. An end user will pay more because the domain is relevant to their business, product or general industry and they see commercial value in the domain.
You have to valuate each domain differently, depending on who it might be relevant to and the popularity of the targeted industry as a whole. Similar sales data can be a huge help here, but also do a bit of research on the subject field too so you actually understand what you're pitching and who you're pitching to.
Not an exhaustive run down I know but hopefully a useful start, there are also plenty of threads you'll find useful on this forum. I suggest taking some time to sift through some for the juicy noms