I've personally sold thousands of dollars of equipment on local craigslist and Facebook marketplace. Never used eBay myself, never really thought it was worth the effort. Price things to sell, and they'll sell.

Stores are most likely going to give you at best 50% of the current retail price, and if it's something "vintage" or not sold anymore, all bets are off. The guitar shop I used to work at gave up to 50% at the most for trade-ins (but that 50% came off the tax too). Add-ons or modifications had zero trade-in value - so if you traded a Squier Strat that was upgraded with all Fender US hardware and custom wound Evans pickups or whatever, you'd get the trade-in value of a stock Squier. The upshot of trading in at a store is that it happens now, not 2 months from now when your GAS has faded and someone on craigslist finally offers you 60% of your asking price.
I work here