Funky do you use Facebook? there is a group on there i've been a member for around 3 years
https://www.facebook.com/groups/322264191317005/
It's called PC and PC parts for sale UK
If you put in your budget on that there will be plenty of people giving advice or have things for sale
You might hit up a 750ti for £50 Sales are usually done through PayPal goods and services so you are covered with PP protection.
I've sold a few things on there over the last few years. My most recent a faulty PSU for £7 they want it for parts so you can pretty much sell anything