While I have responded to requests for items to buy ... I have not been able to sell anything on site ... I guess my stuff hasn't been so popular ...
... requiring a certain number of posts is not such a big problem, or overbearing ... I also agree a percentage of each sale is not unreasonable to request for the site.
But ... be cautious about implementing rules that are too strict ... life always has a way of catching up to us at times unexpectedly so that we may not be able to be active on the site as much as we'd like ... and for that, we get penalized? Just something to consider.
That's my three cents ...