Thanks Paul
Here's the deal. It is illegal to sell mod chips on ebay, but there can be ways around this. I offer all the below at your own risk.
1. Don't put mod chip, mod, chip, or the name of your mod in the title.
2. In the description when making references to the mod, spell it incorrectly such as $box m0d or something to that effect.
3. Don't sell the mod with your ps2. Say you will through it in for free if you win the auction. Things like this are done all the time on ebay, such as saying this auction is for a blue balled point pen and for free I will through in a copied vhs or in your case a mod chip.
Even if you follow the above, there are no guarantees. I tried to sell a modded psx before I knew the tricks above and got busted. All that happens is they will pull the auction and ban you for 30 days. No biggie. Worth a shot. If it's a no go on ebay give yahoo auctions a try. They aren't very good at catching these types of things. Good luck.