1. They are troll accounts
2. They are scammers who would bid up to exorbitant amounts, then PM you to conduct shady business on PAyPal
3. They have the same items for sale, and want yours out of the market
I too have this issue with stuff I occasionally sell. eBay doesn't prohibit blocking people with zero feedback. Kinda makes sense, since they have to start somewhere, but it should be with Buy It Now items or something else. I don't want to deal with newbies or potential scammers.
I use Buy-it-Now, instant payment, no offers.
I've had almost zero problems, with hundreds of big ticket item sales
Ebay has gone from a tragic 'rip-off Grandma and Grampa, scammers heaven', to a rather safe market. It is, however, heavily biased towards buyers, which is a mixed bag.