eBay SDK for Java is a tookit for rapid application development. It does not implement thread-safe methods. The responsibility to synchronize access to any shared object of SDK, is passed on to the programmer.
However, you can easily develop applications such that synchronization is seldom required. The flow diagrams given below shows two possible scenarios. There are two users UserA and UserB. UserA wants to list ItemA and UserB wants to list itemB. Diagram-1 shows a scenario in which it is possible that itemA is listed to UserB or itemB is listed twice to userB.
Diagram-2 show a scenario which is always safe. The items are always listed correctly.