How much would a used Canon 500d sell for?

I don't like it and I really want a Nikon D3100

Not a lot really!

This is an excellent camera - I know I own one and love it.

Be that as it may it depends on its age, condition, lenses, cases and lots of other things.
Personally I never buy used but some people do and have excellent luck with them.

If it's the auto focus feature in video you're after, trust me, you'll be disappointed. I have a Fuji camera that does auto focus in video and it's terrible and I can't turn that off… The focus constantly jumps around trying to find something to focus on.

But good luck anyway

Depends on the amount of use and the overall condition and where it's advertised.

I'd expect to get between £300-350 body only for it if you sold on eBay, probably £400 or so with 18-55mm kit lens.

The other question is what does the D3100 have the 500D doesn't, they're both system cameras both systems have more or less the same lens lineup, both have entry level ergonomics and they're both pretty competent as cameras.