Painting or drawing a celebrity and selling that piece may be alright because once that person becomes a public figure the become public domain.
Now what you use as source material to create that likeness could be a problem because the copyright of that source material belongs to the photographer or artist who created it. Also the celebrity may own the copyrights to the source material being used. So they may not be able to sue and artist for creating an image of them but the celebrity can sue the artist for using copyrighted material to create that image.
Here's a link that helps to explain copyright laws.
http://www.copylaw.com/new_articles/copy_myths.htmlPersonally I've used plenty of images as source material over the years and generally I get permission to to use the images before I begin. I've found that other artists and photographer are generally pretty good when they are contacted first.