Cant Sell In Other Players Shops

Ok i think ive heard alot about this and im having this problem, i have searched but never really seen an anwer to this problem. Ok when i enter another players shop and look in the box i put items into sell its grayed out and i cant place items there, its not a file problem my client says its on patch 29, and ive verifyied my files. To help you understand the problem a little better ill include an image of it. Any incite would be helpfull. Thanks Again.