Right Ping is not a great tool to measure latency as it only measure the round trip time from you to the server and back. Ebay may not respond if there severs/perimeter devcies are set to block ICMP packets (which make up the Ping command).
The reason you stuffer from latency can be a huge number of things, but the most common ones are either a large amount of traffic on your ISP's network or a server which is under a large load. Stopping any downloads etc will help, but if the ISP's network is taking a pasteing then there is not much you can do. One other trick is not reduce the number of deives between you and the server so if you are going though a router at the moment then try just just using the modem and going straight out if you can.
Russ