I don't want to get too far off the topic of MergedMining, but my personal opinion on this (I really don't care much either way) is that the fee should only be charged to 1 party, but not limited to 1 side of the trade.  The person providing the liquidity should be able to make their trade for free.  For example, if I put up a buy order at .15 and a sell order at .16, but the stock is actively trading at .155, I should not have to pay fees when my orders are filled.  The person who chooses to fill the order (buy at .16 or sell at .15) should pay the fee.  This would encourage more open orders and improve liquidity.  Just placing the fee on one side or the other seems to encourage holding or discourage trading, while free trades for providing liquidity would encourage more people to list orders, thereby encouraging more trading.  It would also allow companies to list their IPO without being victim to double dipping (charging to create the asset and then charging to sell the asset).