You can set it up like an E-bay for martial artists. Supply companies could post their inventory lists, or individuals can post items that they are trying to sell. "used" or "new" should be specified.
Costs:
Suppliers should have to pay a fee, depending on how many items they plan to sell in a given year. There should be no fee for the 1st 10 items, $25 Annual fee for up to 50 items, $50 fee for over 50 items. This way you make a seperation between the "little guy" who isn't a supplier, and who may only be trying to get rid of 1 or 2 items, and the "suppliers" who are profiting substancially off their merchandise, and who should be required to pay a fee for your service of bringing them business.
Then, MartialTalk Supply (or whatever you decide to call it) should get 5% for brokering the deal, with a minimum $5 per transaction, paid by the seller of the product. "A transaction" would be one ordering, not one item. So, MartialTalk wouldn't get $5 per item, which could be outragous. If someone ordered 20 items at one time from 1 supplier, it would be $5 for that one order, or 5%, whichever is higher. The seller/supplier can adjust the price accordingly if needed to cover costs.
You need to get some $$ out of setting this up, but it's a good idea. Everybody wins. MartialTalk gets $$ for it's service, Suppliers have a vehicle for selling their products, Consumers have a place to go to find products they need, and Consumers also have a vehicle to sell some of their used merchandise. The fee's I don't think are outragous for the suppliers/sellers either. So, they shouldn't trickle down to the consumer too badly.
These are just broad suggestions and ball-park figures, but...What do you think?