On Classfy right now you can add your forum username and the url to your profile. There is no way to authenticate the user but if many members from a forum join and list with classfy. We will arrange something with the owner of the forum to authenticate the users.
I have added a reddit authentication system too.
Pretend that I'm arn, running MacRumors. Why should I switch to your service?
My own "Marketplace" forum is working well. People who sell a lot on there have a reputation for not ripping others off. For those that are new, it's either caveat emptor or going through PayPal when something bad happens.
There's also Heatware already. It allows you to leave feedback for forum transactions. Your account is linked to all your forum accounts, so if you're new to one forum, but established in another, you have references. Here's my feedback page, for example:
http://www.heatware.com/eval.php?show=to&id=69703&nu...
They have around 70K registered users right now. If you want to displace them, it's going to be difficult (especially because people have momentum on Heatware already: they don't want to lose all their feedback to move to another system).
Heatware is pretty good for feedback, if they facilitated transaction it would have been even better. We are not really trying facilitate feedback of every forum transaction only those that occur on Classfy. However we respect people's previous feedbacks and so You can add your heatware profile to your Classfy profile.
The problem is that launching any marketplace you are faced with the chicken and egg problem. To try and get passed this problem we have released this as more of an MVP. Maybe forum users wont embrace our site but another niche might. So we will pivot towards them. There are MANY niches that need their own marketplace but currently don't. Take a look at http://www.coalmineclassifieds.com/ there is also another one for Oil and Gas and agricultural. Innovation is not evenly spread out in many areas of tech the reason is developers innovate because they want to solve their own problems. How many Techies are familiar with enough with the coal industry in order to disrupt it with a marketplace only dedicated to coal. Which probably will be very lucrative