This will delve in explaining what trespass is and why special attention should be given before linking to any website. First to clarify the case, is that there is a difference between normal eBay customers and Bidder's edge practices. On normal occasions when customers search for products on eBay and they do not find the prices they leave the site to go and look for the same products on other sites.
Bidder's edge on the other side provided a link to all the auction websites and thus all its visitors could compare prices from one site. EBay though it had first allowed this to happen opted to terminate the linking services to Bidder's edge. However, through the use of proxy servers Bidder's edge still had a secondary link to eBay's website. This is now what made eBay file a lawsuit. In the ruling Bidder's edge lost because it infringed the private property rights of eBay.
It is in order to note that causing injury to personal property as well as the interference with another person's enjoyment is considered as trespass to personal property. This was practiced by Bidder's edge when it illegally gained the access to eBay's servers even though the effect was still slightly malicious. In Californian law trespass to computer services and programs is also included as trespass of personal property.
For the establishment of links that will not infringe this law, there should be permission sought before linking to a website. The consideration of own self jurisdiction or intuition should also guide in the relation to the matter.