Lot of good advice here. I agree with a cardboard tube inside a FedEx or USPS triangle, both of which they'll give you for free. FedEx and UPS both sell the tubes for like 3 bucks a pop. The best and sturdiest triangles I've come across were the UPS ones, but they don't sell them at any of the UPS stores near me and they cost actual money, which the FedEx and USPS ones don't. I pack bubble wrap into the top and bottom of the tube, and then stuff newspaper into the sides of the triangle so there's no side to side wiggle for the tube. It's true the absolute sturdiest would be to go get some PVC cut, but that is certainly more work.
I use Pirate Ship, UPS only. I'll never use USPS again, and FedEx is much more expensive.
And I'll echo the above - if anything seems off, just walk. It's not worth the sale and someone else will buy your cue.