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 [it's] worth.
See Rule #1 (
my response to jaybass's "browser" question).
Your rule is an excellent one, but I'll point out that you're skunked if you live by it in this instance.
Judging from your history, your OS will be looong-gone obsolete looong before you're ready to buy a new Mac, and I very much doubt that Apple's business model is to spur sales by making OS X/macOS unavailable to users, anyhow.
I doubt that any of the guys selling on eBay is malevolent.
I'd judge by reading their feedback, and I wouldn't hesitate to take the leap (on a separate partition, if not a separate drive, of course) were I in your position. (As I've said, Apple has created and, in their own way, supported the market.)
