what should i know about leveling merchants?

[MR.hyde]

do i get extra xp for buying expensive items/items that are on sale/consumables?

also, i remember one tutorial telling me to buy all items from a shop because they can have other abilities, now i am very closefisted and the idea of buying 3 pairs of "battle boots" when i already have 3 pairs of those boots in my inventory makes me uncomftable xD how rare are gald? should i really buy all the items from a merchant?

[Simon]

There is no need to buy everything from the merchant. Doing any transactions including buying or selling will let the shop to gain exp to level up. Leveling shops/merchant will allow the shop to stock up rarer items.

[{ST}Immortal Nub]

I'd recommend increasing your shop level by enhancing equipment.