How much is the Lenovo y50 Worth?

While your Y50 technically may be in working order, there are a few red flags:
1. It's 2 years old, which means no warranty (or very close to expiration).
2. It has an 'unreliable trackpad' (your words) and even if it may not matter that much, it's likely going to be a deal-breaker for many.

A quick run-down on Amazon UK and eBay reveals that UHD Y50s are selling for anything between £400 (sounds a bit fishy) and $1,100. Some of these are purportedly 'new', but most are refurbished laptops and covered by some kind of seller warranty, NOT manufacturer warranty. You could try to list it on eBay at £500 and see how people react.

Realistically, I think your options are:
(a) Find a 'Lenovo person', i.e. someone who really wants a Lenovo Y50 (but also knows all about the machine), or
(b) Try to sell it to someone you know, a friend or family member, who trusts you enough to buy a computer that might potentially break down within a few months.

A Lenovo enthusiast might be willing to pay £400 - £500, while an acquaintance/family member will probably expect a 'buddy deal' - something like £250 - £300. Either way, you're going to have to come up with a very good reason why they should overlook the No Warranty and the faulty trackpad.

I happen to own a 2-year-old Y50 myself, and the warranty expired in November. It works perfectly and shows no sign of software or hardware failure. While I'm confident that it will continue to work for another 2 years, convincing someone else to pay good money for it today is a completely different proposition.