unless you used the trade contract area here on the forum, there is nothing you can do except try more trustworthy players.
Sorry for your loss.
My suggestion is, is to only let players whom already have 5g to buy the cards trade with you, unless you want more of this in the future.
All loans without the use of the forum trade area is simply giving another player your gold, with only a chance they send it back.
As an example i have sent a clan member ~5.5 gold, if they decide to quit tommorrow and not bother to send things back, there is nothing i can do either.