I'll take a stab at this but a lot of it depends on actual condition, unspecified brands, as whoever did the internal work. Just to throw it out there, usually
someone who has the buyer specify the price doesn't want to part with their gun unless the offer is enticing enough. Either the owner places more value on it than it's worth or it's a rare item. In this case it's not the second one.
For the gun itself, I usually recommend around 60-70% of retail; retail being with or without GI's 15-20% occasional discount is up to you. I hate buying used internals since it really depends on condition and expected life. I'd estimate around 50% of the retail cost, more if installed correctly, less if installed poorly. Total, maybe around 400.
Estimated breakdown is as follows assuming unbranded items are on the budget side (jg quality):
scar 270, barrel 30, mock mock silencer 15? mags 20, motor 30, gears 60? piston -free, tappet plate - free.
deans -free, (subtract if soldered poorly..)
moseft - brand? (a cheap mosfet is worse than no mosfet).
Call me cheap but that's optimistically if everything is in extremely good condition. You could get all this for around 550 brand new (with some smart shopping and a 20% off from GI), so I wouldn't be surprised if he got offers in the 350 range. Why anyone puts Prometheus gears with a very torqy motor with a stock g&g spring is beyond me..
There's always the 'unknown factor' to consider when buying used guns. If you have no idea who the seller is, how he treats his guns, or who put new parts in it, that's a substantial reduction in my book. I don't even consider paying for 'upgrade time' for someone who is not a quality tech. This changes significantly if you're buying from a trusted seller or tech. Anyone who guarantees quality over a quick sale is worth much more.
As for the electrical stuff, the lipos are worth $15 tops used for all 3 together. A cheapo voltage tester is worth $1 new.. so used would be give away. A charger used twice is still used so maybe $20 tops. The accucel 6 can be had new for under $30 shipped with adapters included and the batteries go for $30+ shipping. No mention of a power supply which is required for the charger. I never understand the 'brand new but used' line. Sounds like he might also give you the 'I paid $700 in upgrades plus a $400 gun and I only want $700 for all of it' line. But that is for you to find out
I'm sure others have their own take on this especially since I don't buy much used stuff and I definitely don't buy anything that uses an m4 mag