The trouble is that used machine vary widely based on a lot of things: Seller awareness, market conditions, buyer motivation, etc.
A bargain for a machine like this would be around 4500 nowadays. It has shocked me how much mills and lathes have gone up in price since the pandemic. I think a median price for a machine in excellent condition with a 42" table is about 5K-5500, but if it has a lot of tooling and really is 'like new' condition, it can sneak up to 8K. This one has some telltale signs it has very little use: check the pan under the knee, look at the bellows, even the table is untouched. It earns pretty good marks for a 9 or 10 year old machine.
I use these proportions in thinking about prices for mills: brand new condition, < 5 yrs old: 2/3 new price > 5ys old and moderately used, around 1/2 new price or a little less. Needing any work beyond casual cleanup, about 1/3 new. Broken parts, or obvious fixes needed, about 1/6 to 1/7 of new price. If a machine is over 25 years old it goes into the 1$-3000$ category, and each one is a special case.
Mill accessories, used should top out at 50% of new regardless of condition. It is a very small buyers marked for mill accessories. I've made an 'insulting offer' to people, only to have them come back to me months later asking if I were still interested.
I hope this helps
p.s.
Around 2 1/2 years ago I advertised a real BP mill with a 12X42 table (yes that is weird) with a single head but a head configuration that could accommodate 2 more. It was 55V 3PH (but motors are changeable) He originally asked 500$ for it, moved to 1000$, but finally had it hauled it off for scrap. I surveyed the machine, and the ways and lead screws were good, but no one wanted it. not one call. not even a low ball offer.