There is a playa trading post on Tribe, bartering and gifting, no selling.
Technically, the commerce which is allowed/flies under the radar: tickets, and fundraisers for theme camps as listed in regionals and events. Rideshare if you count offering to pay for gas and expenses when that section is in operation. If it does not originate with the seller, then recommendations of where to get XYZ for your camp/art/vehicle generally are okay.
I'm too jaded to think up a good sig-line. Need more coffee.