There is no rule. Any attendee is free to barter!

I will say in my experience (and second-hand experience with hundreds if not thousands of participants), that there is literally zero* bartering. And, furthermore, many (cf ~all) people would be aghast at any sort of quid pro quo exchange.

Read the last paragraph from the parent stating they would not return without a large presentation/gift. This captures the spirit/zeitgeist/culture of burning man!

* I have seen some bartering with illicit substances to prove one isn't a law enforcement officer, but the barter amounts to "show me you have some money, even a few cents, and I'll trade you immeasurable money"