What's a good place online to sell $5 art commissions?

$5? Seriously? You might as well work at mcdonalds if you want that kind of money. Consider raising your prices, you need to have more faith in your art.

You can try deviantart's forums, or make an auction on e-bay. That way you can start at a low price, but possibly make more as people bid.

I'm with "Andromeda" on this one. Frankly, if I were a serious client, I wouldn't TRUST an artist only asking $5 for their work.

Even the cartoonists at fairs and tourist spots charge a LOT more for their quick pieces.

If you are serious and need money that quick, ask around at school or church if anybody can use your services.

About thirty years ago, I networked a series of jobs for Dungeons and Dragons players. I would draw up their characters in a series of quick sketches and, for about twenty bucks, I'd produce a nice pen and ink drawing on nine by twelve inch drawing paper. Twenty dollars, and THAT was thirty years ago!