Personally, I would be reluctant to purchase any powder from an individual, particularly a person I do not know very well, unless the container was in pristine condition and had never been opened.
A single canister of powder on the resale market is probably worth less than the hazmat fee charged to ship it, which can only be done by common carrier (FedEx, UPS, etc) via ground delivery.
First recommendation: If the powder container appears to be in good condition and you can use the powder in something you reload for, do so.
Second recommendation: Propellant powders make very good fertilizer for house plants. A tablespoon or so can be very beneficial in a pot of petunias.