"I dont see any cons."
Then to be quite honest you didn't think about it much.
Try the POV that you are an entertainment business and you add on a 4sale/WTB forum for your customers convenience. Don't forget that your boards must be open to the general public (HTC's are) and so in reality any Joe, Dick or Shyster can (and will) register to sell on your site.
When the problems occur (and they will ofen enough) and your customers complain to you about getting ripped off or are unsatisfied with a deal that transpired on your hopsetd for sale forum what are you going to do?
Imagine how long your forum wil remain up when your customers demand YOU resolve issues or lose thier business. Believe me, "buyer beware" is the last thing your customer, who is paying a monthly subscription for your entertainment service, wants to hear.
There's plenty of buying, selling and swapping that goes on anyway. but it is not endorsed by HTC and people will take it to email to bargain. I've bought gear from folks here myself.
With eBay, Craigslists and several other venues available I don't imagine HTC wants the nightmare associated with hosting an explicit 4sale forum.