Ebay not only tracks your user ID, but also your IP address. So a seller who sets up a second account totally separate has to use a different internet connection, such as taking a hike to McDonalds with the laptop, to do his shill bidding. If he did it from the same IP address as his main account, they would spot it in a heart beat.
It is possible however, but as already noted, they have to pay the auction fees when they "sell" it, not something they can afford to do often.
Shill bidding used to be rather common, not any more, as Ebay has more technology than the users do.
I find lots of little deals on Ebay. I needed male and female L14-30 twist lock connectors for generator cables I am making. Home depot wants $20 for a P&S male plug. I bought 4 of them (a real auction, not a buy it now) for $25 including shipping. I found the Female ends in a Bryant brand for $9.99 each including shipping. Home Depot wants $40 for these in P&S brand, similar quality.
I wanted a pilot light light switch for my shop ceiling fans to help me remember to turn them off. Found a Leviton 1221-PLR for $9 including shipping. This is a $20 or $25 switch.
Lots of little things for good prices if you spend the time to look.