I lurk around the Art/Texture board occasionally, and was eventually led to the conclusion that once you sell a texture, you (the person selling it) is no longer allowed to use the texture.

Is this correct? I just want to know before I sell anything (if i ever do).

(I hope I posted in the right board)

Yes it's correct, selling it would mean you don't own it.
You can't use it if you don't own it.

Even though it's just a flat image that I can make a thousand copies of?

Yea. It doesn't matter if you own the file, if you sell it you aren't allowed to use it.