In addition to the above, if a seller sells a lot of high-end, scam prone items, that most people only buy one of (eg TVs, sound systems, computers) they'll probably be less  and more  if someone came along and purchased 10 of them. 

It would prevent bid shielding of multiple items, too (probably more useful back when sellers could have multiple auctions for the same items listed / visible at once). 

Buying limits has nothing whatever to do with shill bidding.  That is when the seller tries to increase the price of the item by bidding on their own items or having a friend/relative bid for them.