
it's possible that you are right but I think (and hope) he was able to see through the bugs and flies since when he was seeing it there was a fly next to magic circle like we see here: http://www.mangago.zone/read-manga/the_duchess_with_an_empty_soul/nbt/mpn_chapter-2269496/pg-16/

That's plausible. There is a higher chance that he had smaller bugs flying around or had planted bugs inside the people. When you look at the bug replaying when the auction house guy died, it's from his perspective.
Which means the bug had to be flying right beside the auction house guy's eyes, or he is inside the auction guy and recording everything the auction guy sees.
I believe it is the latter, because all the people who were captured "died mysteriously". If he had put bugs in all of them, then he would (1) know where they are and what is happening to them at any time and (2) be able to kill them off at any time so they can't tell anyone about him.

But to be totally honest I still hope the new guy doesn't have a bug in him and is not an unintentional spy ⁄(⁄ ⁄·⁄ω⁄·⁄ ⁄)⁄ sorry this has nothing to do with theory I just don't want it becasue: 1) that poor guy already went through hell and I don't want him feeling guilty when he finds out about bug in him making him betray his new master (buy giving info about them) even though not on purpose ╥﹏╥ 2) new guy is an alchemist and we know that mc needs an alchemist for making that potion to increase mana ( http://www.mangago.zone/read-manga/the_duchess_with_an_empty_soul/nbt/iur_chapter-31/pg-85/) so if the new guy has a bug too then the villain will learn all their research as well (if they don't find out about the bug in him before) and 3) because that makes a better story. The good guys are already in a worse position because they don't know the villain's plan, so they are "fighting blind" (don't know villain's next move) and if the villain will also know their every step before they even make it (when planning, through the new guy) then it is pretty much impossible to win. yeah yeah, I know the good guys will win at the and, but still, it is more believable that way to me but again that is just my preference so you can just ignore that part ╮( ̄▽ ̄)╭
I went and re-read the manhwa and the scene that villain sees of mc in the last pg of ch37 is the same as the kidnapped kids see when mc saved them: http://www.mangago.zone/read-manga/the_duchess_with_an_empty_soul/nbt/mpn_chapter-2268574/pg-15/ and as we know those kids died later http://www.mangago.zone/read-manga/the_duchess_with_an_empty_soul/nbt/mpn_chapter-2269496/pg-8/. so the bug in people giving access to information (what hosts see and hear) and being able to kill host is even more probable now(๑•ㅂ•)و✧

The reason why I think this is because the hooded figure set the auctioneer up with all those eyes. even though the auctioneer said that Tigres were only worth their eyes, he kept one alive. Why? He could have collected those eyes that are more valuable than the person.
It all seems like bait. An elaborate trap set up to lure the duke in since his servant is also from the Tigre clan. It would make sense that he’d take in this poor abused person who’s lost everything. He’s a perfect unknowing spy. After all, no one knows exactly how those kids and auctioneer died. But the hooded figure said something about his/her bugs. So it stands to reason the hooded guy who gave the auctioneer the bugs, the tigre eyes, and the tigre, would have placed bugs in the tigre.
If not then the hooded guy isn’t that smart

We know, both @Happyfish314 and I have already said that in our comments. We see the image of mc through kids pov and it was shown he has heard the voice of head of auction house from his pov, all of them were killed so the theory of bugs enabling the villain to get information (what hosts see and hear) and to kill host is pretty solid. And as you already find out, yes, they kept the new guy alive because he is handsome and so worth more alive then just his eyes. Of course it is possible that the new guy is also bugged, it makes sanse from villain's pov, but we are saying why it would be better that he isn't from a plot's pov, as then villain is simply to OP and impossible to defeat. There is a possiblitly that head of auction house kept the new guy alive because he decided that was the way to earn more even though he should only keep the eyes but despite him being greedy and being able to do that I don' think that is very probable since the villain looked in control of the situation and if that was the case he would find out the head's disobedience immediately through the bugs. So yeah, I agree he was kept alive most likely by the villian's orders and so therefor is a very high possibility that he is also bugged. That still doesn't change my personal preference or hope that he isn't. I stated in my comment that this is just my wish and you can ignore it since it just my opinion and has nothing to do with theoryヾ(❀╹◡╹)ノ~
Looks like the bad guy now has a ‘spy’ in the household. He’s using some kind of magic to look through the new guy’s eyes whether the tigre guy knows it or not