I know this is a silly question, but would I be allowed to sell designs or trade designs with people for projects at all and where would I post about designs I want to sell?

i dont think this could be controlled to the point where it would be viable for anyone. See a design could be offered to the person. Could be changed slightly, you could go at them for not paying you for a design you made and technically its not your design. There is nothing the admin staff will/can do as the designs are still unique. On another note, one persons "amazing design" could be someone else's "dishwater rubbish" you spend hours designing for a person only to be told that they dont like it and wont pay for it

In short design contests are kinda like this. The winners are usually given something in return

ok! thats really well said. thank you for answering!