I have yet to sell anything particularly valuable, mainly because I keep such things for my own collection, but I am flabbergasted when I see stamps going at auction for nearly 100% of CV, on NYstamps for example. It's all about demand, rarity, and simple exposure. I've seen circumstances where NYstamps has stuff for auction that can be found elsewhere for probably half of the final auction price. If you find some rinkydink seller who has good stuff but who has yet to really be discovered (or even better one who doesn't know what they have) stay with them until they're discovered and tell noone of your secret.
Edit: something did come to mind about CV. I have Nicaragua 8-12 or some such set listed in Scott for about 65$ for the set. I saw it sold last month on eBay
for 5$. go figure. I had been planning on posting mine, but if it'll go for something so little I'd prefer to just keep it.