The reasons for this restriction are many, but two are more important than the rest.
- It's unfair. Anyone who knows you as a seller has an unfair advantage over other buyers in that they may know more about the item, its history and its value than others. They also may know in advance when it is coming up for sale, and may be able to count on your phone call to tell them when they've been outbid.
- It opens the door to bigger injustices. Though many potential shill bidders will explain that they just want to innocently bid on their family or friends' auctions, allowing the practice opens the door for nefarious sellers to have their family and friends bid merely to raise the bid price and to cost other buyers money—without ever intending to sell the item to family and friends in the first place. More to the point on eBay specifically, shill bidding opens the door to sham transactions—in which there never was an item, and two family members are merely winning each others auctions to be able to leave positive feedback about each other, thus increasing their ability to commit fraud in the future.
All sellers should beware: eBay does not look kindly upon shill bidding when they see it. When eBay detects shill bidding (and they always seem to detect it), both the seller's account and the account of the shill bidder are subject to suspension or worse.