What you are suspecting is called SHILLING:
It is illegal.
It is against the rules.
It is impossible to do so with the selling ID, and eBay also blocks bids from other accounts that it has tagged as being associated with the seller account (including some accounts that have been wrongly tagged as such, at least according to those who have posted here after having been so blocked).
eBay also investigates reports of shill bidding (there is a Report item link on each listing, and "Seller is using other accounts to inflate item price" is in the menu it leads to) to some extent and has other software in place to try to find shill bidders and does take action against them, though this is shrouded in secrecy so we have no idea how successful it is.
But it still happens, more often than it should and less often than it is suspected by some (most posters coming here suspicious of a particular auction turn out to be misunderstanding how bidding works, how bid history is displayed, how other legitimate people might bid, or especially how many transactions fall through with no shilling involved and are then relisted). Trying to decide how to bid based on clues that may or may not be accurate is an exercise in futility: if you set your level of suspicion required to act (not bid) high, you'll get false negatives and you open yourself up to being shilled by that seller; if low, you'll get a lot of false positives, and miss out on some good deals, in the middle and you'll get a mixture of false positives and false negatives so it's still unreliable.
The best way to protect yourself against shilling is to wait until the last minute to place your true maximum bid, which you have calculated (ideally hours or days before; I recommend using a "snipe service" to place the bid--there are reliable and secure free ones) without regard to the prior bidding on the item. If you don't let prior shill bids affect YOUR bid amount, and you don't give the shiller time to probe YOUR bid to base HIS bid on the amount of yours, the price you pay if you win will be based on a legitimate underbid or on the seller's true minimum within an increment or so.
