I don't think this is such a good idea either. I understand that it costs money to run the site on top of all the good reasons for protecting buyers, sellers, etc.
$36 every 10 days is pretty retarted. I think you are going to lose some good sellers in this move. I like to read through entire threads before I make a purchase. I don't want to go around thread chasing for closed threads to see how peoples products are working and to get answers to questions. On top of that you have sellers selling products for $5 on this site, how are they going to compete with a $100 overhead every month to have a forum thread?
I went today to look at a thread I was following and poof its closed. The thread is a way to contact the seller. What happens if they don't want to pay to renew? Now I can't leave a message for the seller on his thread which may have been helpful for other readers. Instead I have to wait for a new thread to open which I have to dig for since the last thread is closed and I have no idea if the seller will even open a new one.
Not trying to sound like a douche but this just makes everyones interaction here a lot harder, prices are going to go up across the board to compensate and you are going to end up losing buyers.
If charging your user base for posting in a public forum is going to happen then I suggest you streamline your payment structure a little better. New threads every 10 days is going to be a nightmare. Set up a monthly, quarterly, yearly plan for sellers that is reasonable. I don't think it should be $100 a month to sell here, it should be $100 a year.