.uk should be absolutely fine. Nominet (the UK registrar) has been around for decades, and is a member-owned not-for-profit company, which is probably why the domain prices are comparatively low.
But whilst there aren't particularly strict rules on UK domains, using a .uk domain name if you have no connection to the UK would be a bit weird - much the same for any other country's TLD.
You'll probably want to avoid Russia (.ru) for obvious reasons.
In terms of the shiny new-fangled TLDs like .win, .xyz, etc. etc. - they are prone to being caught by spam filters. That's not to say they're going to be always or automatically blocked, simply that they will potentially get a higher spam score than 'traditional' TLDs. Note that this seems to apply to even some domains which have been around for a long time, but are still not 'mainstream' (things like .biz, for example).
Of course, all of the above might be irrelevant if you're not planning on using the domain for outgoing mail.
If people are willing to pay your prices, they're not unreasonable.
Some people will only pay bargain basement prices; others will pay top dollar (or £ or EUR) for a high quality product or service.
The only thing you did wrong here is not being transparent with them about your pricing at the outset.