1. Item out of stock. One of my retainers selling it for 100k, another of my retainers selling it for 90k (the price I decide I'm ok with). Someone comes along, sees a 10k difference, panics and grabs the 90k item asap before it's gone!

2. Plenty of items on demand. One of my retainers selling it for 100k (next higher entry is perhaps 101k), another of my retainers selling it for 99,999. Someone comes along, grabs the lowest by impulse and perhaps the 100k too cause.... 1k for the next is still a 1k more. Leave it to their brain to do the "logic".

3. Of course, there's people who forget they've listed the same item on their other retainer (or even same retainer cause.... scrolling), only check the lowest current price and undercut (themselves) by reflex. Or.... they just don't care.