I agree such things are situational, everything in life is.
I do make my living with my tools and I have everything there pictured already, though not necessarily snap on, so I don't need any of it, or even have a desire for it.
Granted I don't do the heavy end of the work anymore where those tools would be most useful. Nowdays I mainly rebuild transmissions and diagnose/troubleshoot for the other mechanics if I have to.
Still I have an idea of their actual value and what they likely cost him. I also know what kind of situation I would have to be in before I sold my tools and they would be the last things to go.
I gather from your wording he is storing the tools at your house, plus he walked away from his job, and they don't look to have been used a lot.
I'm guessing he still owes on them, the snap on ones anyway.
In any case, I don't need or want any of them.
What I would do is tell him to take them around to local shops and try to sell them there.
Better for him and for me.
I would give him a rough estimate on value and let him know what I would pay for them if he can't sell them for a reasonable price.
5 racks of sockets, worth $100 each and more for the universal set, at least $150 there. So $650 in sockets. This is assuming he has the missing ones from the sets, if not they are worth quite a bit less.
I'd give $300/$325.
Extensions both sets worth about $100ish, I'd give 50 or a little less.
1/2" long handled flex is worth about $200, I'd give $100, though I have a new one I paid $250 for not that long ago.
3/8" is $125, I'd give $75 for that.
Screwdriver set I'd give $50 to $75 for looks like there are a couple extra, and this is assuming the set is all there and none are broken.
Channel lock set is all there, that's worth maybe $50ish to me, but I bet he can get $75.
The rest of the off brand stuff, say $50.
I'm not a big fan of snap on air tools, and as mentioned earlier how the impact looks will have a big influence on its worth.
I'd give $700 plus whatever the impact is worth, figure $100.
So something like $800 would be tops for me and I believe I could resell everything in a week for that. All I would have to do is lock my box to keep everybody out of it and post a for sale list on the lid, lol.
And he should have no trouble selling them for that, especially if he happens to have the receipts for them, or if he knows people in the field. Just pack them in that civic and off he goes.
Although if he came into our shop wanting to sell them, he better have the receipt or be known to somebody, otherwise I'm likely to chase him off pretty fast, but I'm guessing a lot of shops aren't that way.
So as a diy homeguy, offering $600 seems pretty reasonable of you.