I do have one theory as to how a potential buyer can watch an item without the seller realising, although I suspect it's as a result of an eBay glitch.
If somebody has viewed the item recently but has not added the item to his/her Watch List then so long as it is still showing up on his/her Recently Viewed Items page that buyer can still end up getting offers to buy the item. I know this because I was searching for a particular item about a month ago and saw a listing that looked interesting to begin with, but upon clicking on the listing and looking at the photos closer I changed my mind when I saw shots of damage to the item and decided not to add the item to my Watch List. I subsequently continued looking for the same kind of item, but around about a week later I received an offer from the seller to buy the item that I'd looked at but had not included on my Watch List. Therefore, although eBay's intentions may well have been good with regards to giving sellers the option to send watchers an offer, it appears that offers are also getting through to users who looked at the listing but have decided not to add the item to their Watch List.
As a result of the fact that people who have looked at the item but have not actually added it to their Watch List can actually end up receiving offers from sellers, these offers are, in effect, nothing more than an eBay generated version of spam mail when you've already decided that you're not interested in the item, yet you received unwanted offers from the seller to buy the item. eBay should sort out the glitch in their programming and make it so that the offers only get through to people who actually have the item actively showing up on their Watch List, and make sure that the offers cannot get through to anybody who did add it to their Watch Lists but changed their mind and deleted it, or who looked at the listing to start with but decided not to add the listing to their Watch List.
