My concern is that the relase would hurt forum activity but i can see users posting bargains on the forums to give users a incentive to visit their forum posts. EX "offering these items at 20% cheaper then the markeplace value"
I really like this idea! It would be very neat to see it implemented into the game! As far as slowing down the forum posts for advertisements I don't think it would affect it to much I could still see people utilizing them to let people know they have stuff for sell as still another form of getting the word out there of their wares.
I like the idea of a marketplace. One thing I think that should be considered is to limit the number of different items that can be placed in the marketplace. This might prevent players from listing everything they own in the marketplace. I don't think there needs to be a cap on the quantity of any item just how many items can be listed for sale. For example, say someone has 15 different items of varying quantities listed to sell and 5 of them sell out then they could list another 5 different items with no cap on the quantity of each.
A FB game has something similar, you post an item for sale in the auction house, you can put a time limit on how long it is for sale, a min/max price , or a buy it now price. Also if you click on the picture of the item being shown you can see it in an empty room, or on your nekkid avi (then option to see it with the items your wearing). I think this would make it so much easier to find things you want and to see what they look like on your avi. Many clothes items I love on other people avi's but hate on my own.