I've often traded in gear. You get less as a seller because the shop earns their cut selling the guitar. Nothing wrong with that. Sometimes it's worth saving yourself the aggravation.
You definitely do better on Reverb, regardless of having to pay taxes, or on a private site like TGP. You lose 25-30% on a trade-in, and taxes are far less than what you'd lose selling to a dealer (which is essentially what a trade-in is).
The reason I've done trade-ins and sold to dealers is simple: You get paid, you don't have to deal with the customer. Sometimes that works for me, sometimes not.
May I be the first to say that the ideal thing is to hang onto the guitar if it's a good one? I have so many regrets about guitars I've sold or traded in.