I should have been more clear. These would not be coins you have to buy. Just coins within the game - you'd get them by selling existing items you have. Rather than directly trading collectibles, you sell them for coins then buy other collectibles with those coins.
if we started up a new game, would you want it to be exactly the same? or would you be interested in changing it up (make it more of a market where you can buy/sell any item for coins, probably more competitive, but you could trade many times per day). I'll probably set up both games in tandem before people have to choose one over the other.