Thank you @Huaai0!
Yes, not everything on Ebay is that cheap anymore, nor Gumtree for that matter. You have to really trawl through listings to find the bargains and sometimes the gems are hidden behind a bad description/terrible photo.
The bathroom basin, for example, just had a title like 'sink' and a dark photo where you could hardly tell what you were looking at, but because I was on the look out for that exact type of basin, I spotted it! That's also why I paid the seller more than she was asking. She was inexperienced and I don't think realised how much she could sell it for. She was so happy when I insisted on giving her £25 instead of £10!
The shower in the other bathroom is another example. Terrible, blurry photo from picture on outside of box, no real information on listing except for a link to a page where you could actually see what it was. I realised this was a brand new, undamaged shower, retailing at £450-500. I got it for £30.
In the end though, it's a question of time versus money. A lot of people have neither the time nor patience and would rather just pay retail and have it done with, but because I had a big house to fully renovate on a tight budget, that wasn't an option. Anyway, I just LOVE finding a bargain, so the 'trawling' is an enjoyable process for me. You just have to be careful not to buy lots of random stuff you come across while you're doing it!