I'm not a card seller very often so this won't really impact me, but I think its always awesome when you provide a better service (or offer it for less of a fee). So as always, great job Peakmonsters!
On your DEC question, I think you should not feel you will kill the market. You might want to use limits and be strategic about it, but there is no reason you can't sell into the market to pay bills, pay salaries, buy presents, or even buy more monster cards :) ... You don't have to put a market order at any price out there, just set reasonable limits and sell over a period of time to let the market absorb it. Its not healthy for the market if you withhold it either, so in the long run you aren't doing your customers a favor by holding it if you really need to sell it. We all want you to succeed, so don't worry about us feeling like you depressed the market.
Again, good job on the listing for sellers and I hope you get the bids and rentals going too... Both should be awesome!!!