It means don't buy anything you want to be stereo matched in separate eBay auctions. If I need two of anything matched and want to risk eBay, I'll only buy two from the same seller. I also ask if the serial numbers are close and if they believe the two items work well as a stereo pair. That has worked out pretty well.
Also, certain things just don't seem well suited to buying used on eBay from anonymous sellers, and monitors is one of those things. I've always bought monitors new, and even then have had to send one back for exchange.