I was a member at joomlart.com which expired very recently. i have downloaded over 20 templates from there during this period. i've not seen anywhere on their site about trading or selling templates. ( i have not even used 2 of the 20+ i have) so would be legal to trade/sell it in a bid here ?
From Joomla's Terms of Use: "I. COPYRIGHT INFORMATION All editorial content and graphics on our sites are protected by U.S. copyright, international treaties and other applicable copyright laws and may not be copied without the express permission of JoomlArt.com, which reserves all rights. Re-use of any of JoomlArt.com's editorial content and graphics for any purpose without our permission is strictly prohibited." So I guess your answer is no, you can't sell them or exchange them.
I think my warped sense of humour bypassed you there - you're NOT ALLOWED to sell them on, whether you've used them or not. Joomla probably consider downloading the templates as usage anyway, so you have already used them.
oh well then .. .guess i have to use them myself someday . hope these joomla dudes dont have an expiry dates on their templates =))