Me and my buddy both upgraded our Macs from https://ipowerresale.com/. We both had a great experience. That Microcenter is a good deal, but in person only. It wouldn't be feasible for me and probably not for you. I got my 14" fully max spec for $2,600 for in pristine condition. Not that that spec fits your budget, but I recommend their products and service.
If you're thrifty and resourceful, r/appleswap can have amazing steals if you're quick about it.
If you're thrifty and resourceful, r/appleswap can have amazing steals if you're quick about it.