I admit, I put myself in this situation. I created some unique, non-magical items for my PC (with a lot of help from folks on this site). I assumed he would want to keep and use these items, but he has decided that, because they're so unique, they must be valuable, and is trying to sell them. One example is a loose "camouflage" tunic that provides +2 to stealth in forest environments.
My attitude is: if the PC wants to sell the items, that's great! This is his adventure. But it has presented me with the issue of determining these items' value.
I don't think this problem is necessarily specific to my unique items, either. At one point, my PC rolled a natural 20 perception check on a meaningless pile of rocks in a meaningless cave. There was nothing worth perceiving, so I said he found a hunk of unrefined gold jutting out of a rock. I wanted to give him a little reward for the natural 20 (and I'd rather not debate that, because I like to reward 20s and penalize 1s), but how much is the hunk of gold worth?
The question is, how do I determine a fair value for items that I can't find in the D&D items and equipment list? So far, I'm trusting my gut, but it leaves me wondering if I'm being fair to my PC. Any thoughts?
(Update: I really appreciate everyone's answers so far, but responses have been focused on the two specific examples I gave [the tunic and the gold]. I'm looking for general guidelines/thought processes that I can apply to any item that doesn't match anything in the D&D list of equipment. The two I listed were just two examples. Sorry for the confusion. Thanks.)