I am unable to use /buy since I am using BuyCraft on my server.

Possible fix;

Change commands to
/dbuy
/dsell
/dworth
/dprice

Etc etc.

It appears to be a known issue that buycraft overrides other plugins.

I would suggest configuring buycraft to use a different command, even if you were using buycraft solely on its own, but from what I've seen that won't stop buycraft from trying to take over the command anyway...