Whether a proposed system would or would not, infringe a patent can be a fairly complicated question requiring quite a bit of analysis. It really can't be reduced to "Ebay has a patent, can I do something similar to Ebay?"
The "invention" covered by a patent is defined by the claims of the patent (those numbered statements that come first in the uspto full text database, and last in the printed copy of the patent). In order to infringe a patent, the accused system must incorporate every limitation, or its substantial equivalent, set forth in at least one of the independent claims (a claim that does not refer to another claim). Whether the accused system does things in addition to the claim is not relevant to whether the accused system infringes.
On top of that basic principle, there is a wealth of principles of claim interpretation developed in case law relating to patents, which guide the way one reads the claims. Also, the apparent scope of the claims can be influenced by how terms are defined in the patent, or how terms were clarified or amended during prosecution of the patent.
When you get finished with that, then one might consider the validity of the patent - i.e. whether there was one of a specified type of event, such as a publication more than one year before the earliest US filing date of the patent, that would render the patent to be invalid.
