I suspect the strategy is similar to bracketing.
Bracketing isn't uncommon in small financial markets. You basically put a sell order in at a price a bit higher than the market and a buy order in a bit cheaper than the market. As the market moves, you also move your orders. The thing is you can't move them perfectly in sync with the market, and occasionally a large order comes in that blows through a few orders in the deck. So when someone comes along and buys a big chunk your sell order gets exercised. Someone else comes along and sells a big chunk and your buy order gets exercised. As soon as the orders are exercised you replace them.
The key is that you're always buying for a bit less than market, and always selling for a bit more than market. As the market moves you strongly tend to make money, regardless of which direction it goes. The more volatility, the more you are likely to make.
I think the strategy for being the higher priced book is similar. The seller knows it's a rare good, and they're counting on some class somewhere requiring the book. A bunch of students will need it all at once, and the cheaper copies will get snapped up, forcing some students to buy it at the higher price, and boom, your sell order at just above market got executed.
Maybe they should have limits, but it's just one book and it's unlikely to turn much of a profit, so it probably wasn't worth the effort.