Basically you don't want to care what others are selling for because it's almost impossible to compare two trailers as to condition and features.
Right now the buying season is just starting and you should start by posting it for whatever price you think it might bring. Any 40+ y.o. FGRV in decent condition and ready-to-tow will bring in between $3500 and $6000, some even more.
Best selling prices seem to come from eBay sales as you hit a much wider audience. Just put it up with a reserve price and sit back and watch.
BTW: I am guessing it is ready-to-tow, new tires
, bearings repacked and a current title
in your name. Those are all essentials for top sales prices