Are you using the default UI? If not, try switching to it first via the Load UI button on the Options window (Alt+O), General tab. If already using the default UI, try using the U key instead of mouse clicking with the merchant centered in the window. If it still crashes try exiting EQ, delete (or even better rename) the default directory in the uifiles directory of the base everquest directory, then repatching to get a fresh copy of it and try again.
The problem sounds like an issue with the merchant window UI file.
If all this fails, you can try renaming or deleting your UI_charname_server.INI and charname_server.INI files in the base everquest directory, and repatching. If it still crashes after that and only due to the merchant, I'd suggest trying the sony everquest troubleshooting forum or using the online chat feature to try to resolve the problem.
Hope this helps some,
Yther Ore.