Well, celebrities have a right to profit off their own image - the right of publicity - and there could be a problem if a celebrity is feeling especially litigious - but quite frankly I have never heard of a celebrity suing over a
portrait painted and sold of them.
If you used a photographic image of a celebrity that was taken by someone in order to create your portrait, however, the odds might be increased of hearing, not from the celebrity, but from the photographer who owns the copyright to the photo you used.
I also have heard of celebrities stopping the sale of tee-shirts and other products created with their image on them. Paris Hilton, in fact, has created her own line of tee-shirts with her picture on them -

- so I imagine she might be a little more protective of her image than others might be.
It is wisest certainly, in other words, from a legal standpoint, to get permission to use someone's image if you plan to make money off of it - that eliminates the threat of a "right to publicity" lawsuit. But the chances of a celebrity suing if you don't is also probably slim. The worst that would
probably happen is that you may receive a request to remove the item from eBay.
I guess it would be up to you to decide if you want to put a lot of effort into a painting only to risk being told you can't sell it.
Sorry I don't have a definitive answer for you.