To the texturevendors:
If you DONT want to give away your textures for free, upload 2 versions!
1. the original which can be bought through your vendor.
2. A displaytexture for your vendor.
Put a BIG disturbing logo or text on it.
So all everyone can see (and steal) is the brandmarked texture. Useless to steal such a texture.
Remember: Textures are NEVER save in SL. Everyone able to read the cache can steal every texture he can see.