What would you consider a fair deal for a lightly used Peavey 6505 Plus 120W amp head? (not including a foot switch) I was thinking something like 600$ would be fair, but would you go for lower?

Thanks for the advice.
I've seen them sell from 450 to 750 on the used market. A lot depends upon the condition of the unit itself. If it's clean the price tends to be on the higher end. If it is pretty banged up, scuffed up, and crusty... tends to go lower. Market also plays a factor, supply and demand so to speak.
Gear: Gibson Les Paul Studio, Gibson SG Special, Fender Stratocaster, Fender Telecaster, Fender Jazzmaster, Gretsch Pro Jet, Carvin C350, Epiphone ES-339 P90, Epiphone ES-335 Pro. Peavey 6505, Sovtek MIG-100, Vox AC30, Peavey XXX.
I wouldn't have thought of you as a 6505 metalhead kind of guy Mark. Shouldn't you be doing posh spazzy things like ironing your socks?
My Gear:
Ibanez Jet King 2
OLP John Petrucci
Squier Stratocaster (modified)
Harley Benton CLD-41S (Acoustic)

Peavey Triple XXX
Peavey Vypyr 30.

Harley Benton Vintage Overdrive
Boss CH-1 Super Chorus
Boss DD-3 Digital Delay
Boss FRV-1 '63 Fender Reverb