The PPT FFL fee CAN be taxable if the FFL gets involved with finding the buyer or gets involved with the price of the firearm (in which case it is best for the parties to do the transfer at another FFL).
Any transfer fee/FFL fee is taxable if the firearm is subject to sales tax, which the fee is normally higher than $10.
FFL Transfer/Special Order Dealer since 1993.
Net-FFL list maintainer.
Never try to teach a pig to sing. You waste your time and you annoy the pig. - Robert A. Heinlein