Even though there is various tools for editing the database, (EOC, etc.) I use George's Tools for editing merchants, spawns, etc. Few steps to installing it but works for me.
http://www.georgestools.chrsschb.com/
As far as merchants keeping items sold to them, make sure, in
variables table MerchantsKeepItems is set to 1 and in
rule_values table World:ClearTempMerchantlist is set to false