Well, we don't know if it's a scammer / Darknet dealer / criminal or even a hacker himself who's going to sell it instead of an innocent person.
That's kind of my point. Looking at the history of the coins being sold doesn't tell you anything about the person selling them.
Because he's owner of the coins.
Do you know the history of every satoshi you own? I very much doubt anyone knows this.
It shouldn't be any issue because if purchased, the buyer could "spy"* as well, because he'll get to know the address anyways.
Yes, but that's different. If someone is trading with me, then of course they will see the address I sent coins from (although those coins will be either mixed or coinjoined, so they will learn nothing anyway). But someone who I might not be trading with asking me to reveal to them my address(es) so they can perform some kind of rudimentary blockchain analysis on them and then maybe not trade with me at all? I've just lost some privacy for absolutely no benefit to me. So I would pass and find a different buyer.
* I wouldn't call it "spying", "spying" sounds more like with bad intentions, while the buyer can be 100% friendly, without any sinister intent.
But that's exactly what it is. He wants to spy on the coins he is going to receive to see if they are "good enough" for him.
If you are that worried about the history of coins you buy P2P, then just mix or coinjoin everything you receive. Simple.