First thing you should do is change your sessions cookie. Logging out then logging back in should be enough to change your session cookie but you can just change your password. It's good practice to change your password though so I would recommend that.
To further avoid this try not to buy often from usershops. If you have to, then disable your javascript first. This is most likely a way to prevent it.
To enable/disable JavaScript in Firefox 2.xx or 3.xx series
1. Open Firefox.
2. On the Tools menu, click Options.
3. Click on the Content icon. (Fig. 1)
4. Check the box next to Enable JavaScript. Uncheck it to disable. (See image below)
5. Click OK.
Edit by iargue: You can use the programs on here that use the usershops, that is safe.