I've always used Parcel Force when I've sold a guitar to someone on either eBay or a music ad, because I can drop the parcel off with them. DHL, UPS, FedEx and co all insist on arranging a pickup which usually is during working hours when - guess what? - I'm working

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To be fair I've only sold a couple of cheapo guitars that didn't have hardcases to protect them inside their cardboard boxes, but I wrapped those within an inch of their lives in bubble wrap. ParcelForce has always got them to where they were supposed to be, and around here in Norfolk anything I have delivered to me always has a card left. Anything I have sold has always turned up in good nick at the other end too.
The absolute worst things are packages from the US. If they're sent via one of the big carriers (UPS, DHL, FedEx) they get picked up and delivered by the same company in both countries. Everything - and I mean everything - I've ever had delivered via USPS has always turned up in an utterly shite state

This is because once it hits these shores it no longer becomes a USPS parcel and becomes a ParcelForce one instead. USPS don't give a shite because any damage they simply blame on ParcelForce our end, and ParcelForce don't give a shite because any damage they'll blame on USPS beforehand. Moral of the story.....don't buy from the States if the seller wants to use USPS. Pay more for one of the big 3 couriers, or don't buy at all.