I finally have enough data to compile a list of purchasable items.
http://80.237.220.68/files/allowed_vendor_items.txt
This should be enough for now to clean up the existing merchantlist table. Unfortunately I can not do this myself due to a lack of time in the next days.
Hopefully someone other has the time and uses this data to shrink the merchantlist table down. While simply grepping out all valid entrys should be done in a matter of minutes I guess you also have to recalculate the slot id for each vendor.
As soon as my time permits I will start to write scripts to generate the merchantlist table from scratch. On a first glance there are quite some merchants missing or the npc's are not flagged as such.
EDIT
Do the items in merchantlist_temp show up on the merchants? My understanding is that this table is dropped on an restart?
The loot_it seems to start at 10 and not 1 for each vendor.