This is a sort of AUCTION HOUSE feature.

The currency used in the black market is RED TOKENS/BLUE TOKENS for transactions fees and SM for actual bids. (BLUE tokens aren't in SM but perhaps you could have a use for them here.)

BLUE TOKENS are required to put up an auction for Mythical Items(Possibly any higher tier of items should mythical be outclassed.) say 25 tokens to place an item on the market.

RED TOKENS are required to put up and auction for LEGENDARY and below 5 tokens.

As a Bidder, You will have to bid with an amount in SM, HOWEVER, to place a bid, You must pay a fee in BLUE TOKENS, such as 5 Tokens per bid.

Items with a Power Level over 5 cannot be traded. Colored Items can not be traded.

Actually, i wouldn't mind this. You could try to get lucky and get mythical's on an alternate account, but there's no guarantee that you'll even get the item and you have to buy the item outside of SM gold. As long as it does indeed stay as an auction for all to bid on and not a simple means to trade around items. And as you said, could bring in the element of blue tokens more into SM. However, I think the cost might need to be tweaked.