I'd love to be able to trade, but I've never been able to come up with a way to trade that I couldn't abuse, so I've never suggested a method. Your way is hard (but not impossible) to abuse, although that is in large part because you've limited most of the things people want to trade for. I mean, I'd love to be able to trade a player starting out a dozen or two upped cards and 10k electrum to get them started for that nymph they just spun, but if that's possible, then so is creating 1000 alts and trading all the nymphs to your main account.
Of course, that's not at all what you've suggested; your trades are 1:1 and exclude the more difficult to obtain cards. The most abusive thing you can do with your method is to trade upped cards you have extra of and would normally sell for ~1100 electrum to get ones you would need to spend 1500 electrum to upgrade. Unfortunately, that's also the only thing you can really do with it.
I think your system would work a lot better if we had more rarities of cards besides 'bazaar buyable', 'rare', and 'nymph/mark', especially rarities that took considerable skill to obtain, rather than 'oh look, the fat man was nice to me today'. For instance, with that trials project Zanz had been talking about, if you won certain cards obtainable only in that manner, a skilled player could obtain those and trade them away to others who are either too lazy to get them, too unskilled to earn them, or weren't able to get the ones they wanted. (the equivalent of winning 10 morning glories and all the other rares in the reels but no titans when you really want to play gravity, for instance)