And what would I be doing at an Apple Store anyway?
Taking advantage of Apple's generous offer to users in your position, i.e. bring your Mac to an Apple Store and get upgraded free of charge.
OK. So I guess that it's back to square one.
I'll wait until I have to get a new Mac with MacOS Adirondack already installed.
Well,
Apple created a market for bootleg installers when they decided against accommodating people in your position,
and as I've pointed out before,
they don't police those offered on eBay.
Take it for what its worth.