I dont really think purchases not being available to non members is a flaw, more a bonus really if you have good products, then it will be an incentive for people to join up. Doesn't ebay work the same way. That presuming that non members can see your items in the first place.
I also think its a good thing to let members sell their own stuff, just so long as the admin has a delete button for the more unsavoury items and con jobs.