Personally, I am against the selling of customs on ebay as a way to fund your hobby. It's the same as scalping, if that's why you're going it. You're basically taking advantage of people who can't sculpt/paint/etc - the same way scalpers take advantage of people who can't find new toys. Now I know that some of you will disagree with that outlook, but that's my opinion.
Now, you may think that this next statement contradicts what I just said: I don't see a problem with taking commisions from people to make specific figures. How is that any different, you may ask? I look at it like this: I make a Clone, because Clones are hot, and I put it up on ebay, and hope that the bidding goes up as high as it can. Like I said, basically the same thing as scalping - find hot figures, put them up on ebay, make money.
When a person contacts you about making a specific figure for them, first of all it is a customer who contacts you, not you making something to see how much you can get for it. You're also going to give that person a price before starting to create the figure. And if that person is okay with that, then that's okay.
To me, it's just too very different things.
However, if you have been customizing for a while, and just need to get some of your older works and want to put them on ebay, I don't see a problem with that. I know it sounds like I'm contradicting myself here by saying what I'm saying, but it makes sense to me