I am listing some guitars on Reverb, and there is an option to ignore low-ballers when you list your gear. I think the cutoff is 64% below asking, and an offer will not get through. But there is always "Contact the Seller" option, if you want to ask. That has worked for me. And, all my gear has the "Make an Offer" option available, so I can get trade offers. It is amazing how many people will make ridiculous offers.