Possibly more an issue with high speed, cable, Internet than dialup phone modems. Anyone that has hacked your mailbox password, quite easy with common email services, has access to your maximum bid amount; after reading, I immediately and permanently delete that eBay email. Less likely is that your "my eBay" might be hacked and items being bid on examined including maximum proxy bid.
An easy, instantaneous matter, to keep changing eBay password (entering into logbook at the time), if any serious bidding is to be done. A easy matter for unsavoury individual(s), or anyone, to determine when you are bidding and for what item(s). A good case for using snipe bids, further reducing the risk by placing snipe bids very late in the auction.