Craigslist or FB marketplace around here they go for $400-600. Thats for LT275/70R18 Firestone Transforce HT. The AT might bring slightly more, but having driven on the AT on my previous truck, I dont see much added value to the ATs. The $400 set went in less than a day, the $600 set lasted nearly a week.
I have seen the HT for sale slightly under $200 ea (note the HT are being replaced by the HT2 and AT by AT2) and I used to figure 2/3 to 3/4 low retail for such - so $600 is probably upper limit.
Discount Tire dealer I talked to in Illinois (killing time while sister-in-laws truck in for tire repair) offered $70 a tire for immediate takeoffs HT or AT if I bought $1500 worth of Michelins. So if you got $400 from dealer, you probably did OK considering the hassle of transporting and storing and advertising and transporting again and meeting buyer for MAYBE an extra $200.