It's possible to add item to your watchlist without viewing the listing - just click the heart icon on the image from search.
Interested buyers can include buyers that watched your item, added it to their cart, and in some cases just viewed the item.
When you go through the motions to send an offer it will tell you how many interested buyers will receive the offer.
Interested buyers is eBay's phrasing - often people that just viewed the listing but didn't add to their cart or watchlist are not interested, but eBay does not have a way to separate those offers. It's all or none.