It's not rude, it's dishonest.
If you're only willing to buy used gear via private sale, you must also be willing to take the risks associated with buying used. If you're not willing to take the risks, don't buy used gear.
If you're only willing to buy used gear via private sale, you must also be willing to take the risks associated with buying used. If you're not willing to take the risks, don't buy used gear.