The steam marketplace also has algorithmic traders (despite being against TOS to automate marketplace interactions). I found myself in a position where there was only one other listing for a limited time discontinued item that I had bought a few of. I bought the other one so I could set the market price (gouging sure but there wasn't any demand so still risky). Another one was listed within 15 minutes. I bought that one too, steam only reveals to you who the seller is after a purchase (and it was the same seller).

I wasn't convinced it was a bot and not just someone paying close attention to their steam account until I realized they would under-list me every 5 minutes or so. Since there was no activity on that item, and no buy orders, I experimented by listing my item lower and lower, and they kept undercutting me, down to a floor. Their floor ended up being pretty low (around an 85% loss on what they originally bought at), and after I bought at the floor they then re-listed another for double that, before slowly declining again to under-cut me. For an item with no demand, I decided not to try to slowly buy out their entire inventory at the algo's floor.

I found other items that bot was also trading (after I bought them) and discovered that they also change their listing prices based on where people have buy orders set (a good indicator of some demand / market value), so their floor is higher on an item with any demand at all.