[lokizito]

Wrong typing. You should always test the query somewhere before using in SM since than you can be sure of the problem. Also, don't bump.

 len += Format(query[len], sizeof(query)-len, " (`STEAMID` TEXT, `ITEMID` INTIGER, `AMOUNT` INTEGER");

should be

 len += Format(query[len], sizeof(query)-len, " (`STEAMID` TEXT, `ITEMID` INTEGER, `AMOUNT` INTEGER");

And one tip: use SQL_TQuery since with it you can log the error:
Callback

SQLTCallback public(Handle:owner, Handle:hndl, const String:error[], any:data);