Personal checks are fine. Out of state checks clear in a day or two now. Wire transfers will work. Sorry, no credit cards.
UPS mainly. In 30 years I've had one claim and they paid it. I don't like Fed Ex but I'll use them if you want. Don't expect them to pay a claim on a vintage guitar. Proper packing will prevent problems. You got to double box expensive guitars and amps.
I don't buy anything I wouldn't want for myself. It's very rare I'll buy something that's not 100% original, but if it has changed tuner buttons or been refretted, I'll tell you. Detailed, accurate descriptions only. I go to a lot of trouble to pack this stuff right for shipping so it will arrive undamaged. I sell only really nice amps, fully serviced, no return on amps.