Watch out for bugs
There was a case where two sites were selling a rare book. One had an automated process to set the price slightly less than the rival's price. The rival had an automated process to set a price a bit higher than the first's price (probably with the idea that if they got an order they would buy it from the first and resell at the higher price).
Site 1 saw site 2's higher price and nudged their own up a bit. Site two saw site 1's higher price and nudged their own up to retain the margin. Site 1 saw this higher price ...
A few days later the book was priced at some insane value - many hundreds of pounds.