Depends what you're selling. Some things have a good resale value, others not so much, and it's not necessarily based on their price when new. Instruments are one thing, electronic equipment is another—once an item's been out for a short while, unless it's some handmade boutique thing or high-end pro outboard, it's lost a lot of value. With instruments, some eras are more valuable than others—e.g. a 1980s or 90s Fender with the replacement pickups, bridges, etc. fashionable at the time may be worth less than a newer US-made unmodified one, which has nothing to do with quality, just style and trend. Some clever person could probably right now buy up all the metal-band-styled BC Rich, Dean, Schecter, etc. instruments out there for small change, wait a few years and make a fortune when they get trendy again, as 1960s Kent, Baldwin and Harmony have done.