I don't think there is anything that can stop you from doing that.
Some stamp companies require that you use a copyright stamp if you are selling something with their stamped images on it. So as long as you do that I think you are covered legally.
I do know that people have mixed feelings about selling swaps.
Lorien <--My Blog
Giggle Mingler (Hee hee)
"A raω onion ωould pгοbably be rubbеd on unbroken сhildbrainѕ with гeрutablе gaіns." - Comment from a Blog Spammer as Read by me