I bought a used Rem 700 ADL in .270 Win with a 4x Leupold about 2 years ago. If I remember right. I think I gave about $325 for it. Considering this one is a 7mm and comes with quite a bit of ammo, I'd offer about $400 and see what the seller has to say. $500 would be my max so if they don't like $400 I'd go up in increments from there.
For what it is worth, my usual go to rifle is an old Rem 700 ADL that I've had since I was 13. I don't know how many heads of game it has taken but it has been all over North America and has also been to Africa 5 times. I've taken 68 different species of big game and have taken probably 50 of those species with the 7mm. Mine likes Nosler 160 gr Accubonds and Partitions in a handload. I wouldn't hesitate to hunt anything in North America with the 7mm Mag.