i dont like this rule:
 since  there are people who may not buy a membership and want a membership to get higher lvl etc.
so they can only buy it from other people  with gigas.
if you are trading it with gigas.
when you are in trade with the other guy.
take  screenshot of the trade how you do you screenshot? simple  press on print screen and you got a screenshot. 
and how to upload a screenshot? simple go to 
www.photobucket.com.
www.imageshack.us upload you  screenshot there.
after its uploaded take the IMG code and make topic put screenshot and you done.