Quote:

I was able to download the Black & Decker manual just fine from manualls.co.uk, even via a UK VPN...

manuals.com doesn't seem to work at the moment. manualslib.com works just fine, including downloads.

Haven't seen any 'power user' errors when using ebay.co.uk, either (also via a UK VPN).

I can only conclude that your problem is entirely local. Either your ISP got marked as shady and you get more captchas and other checks than I or other users do, or your clicking patterns tend to trigger additional protections on various websites - that'd be a case if you did too many clicks at once, etc. It's likely worth waiting until a website fully loads before you start doing anything on it. In case of manualls.co.uk it takes about 10s from clicking 'Download manual' to getting the download popup. The website does a lot behind your back when you do click on it and it will fail miserably if you click it before it has finished loading.

I'd recommend:
- try out a different ISP, perhaps by tethering on your phone
- patience, wait for websites to finish loading
- reach out to Ebay support as they are the only ones who can help you fix your "power user" problems.