I mean by fellow passengers/individuals not the aircrew. Like if the person sitting next to you offers to sell you something.
There are no such restrictions on flights.
However, for your own safety; you should be careful and apply your normal common-sense checks, just like you would in any other transaction.
There are no specific protections provided to you, just because you are on a flight.
You would still be liable for the sale (for example, if the item is subject to customs you would be liable for that; similar to taxes, etc.).
Especially be cautious purchasing electronic items which may be [a] counterfeit [b] stolen [c] infested with malware.