Of all my vector work (which is 100% of my portfolio), icons and buttons sell the worst. At least the general web buttons and icons. I have some specialized sets of icon graphics that do ok. But if you are talking about typical web buttons (web-related imagery like network symbols, feed symbols, browser buttons, nav icons, etc), the I would say to focus your efforts elsewhere.

Suer some people have great success in this category, but it is an extremely saturated segment of the business, and icon sets take a long time to create. With less than great returns, it might not be worth it.

I personally do better with backgrounds, illustrated shapes and symbols, more graphic stuff and less button/icon types of sets.

I think it depends on your skills... I sell a lot web buttons, I don't think is a saturated market... but my backgrounds doesn't sell and on IS they just don't accept them.

Good luck!