Guitar Nerds Anonymous

Has anyone ever traded guitars in at shops in order to buy a new one? How do you judge/work out what a decent trade in price would be?

Say if you have a semi-desirable guitar that's selling for about 500 on ebay/gumtree 2nd hand & you want to trade it in at a shop what would you expect to get?

The internet seems to be unwilling to yield dealer prices on guitars but I guess the mark-up must be around 50% on big brands - so would it be reasonable to expect store credit for around 750? Despite working with money every day my knowledge of retail practice is incredibly poor...

If it was going for £500, they'd likely offer you £300.