You can change the name of the merchant, but right now only to already present names and it requires a lot of looking up ids in the db. You're better of with leaving it like that until someone releases info on how to edit the client.
To change the prices look inside the MarketResource table in the Arcadia database. There are all the shops listed by name and what they sell. Its best to filter that table to the shop youre trying to edit and order the entries by sort_id. That way you just have to count from the top (or bottom) ingame and in the results to edit a specific item, since they appear ingame ordered by sort_id.
To change the prices you need to change the price_ratio. That value will multiply the price of the item that is set inside the ItemResource table. You can also look up that item in there and change the price there, while setting price_ratio to 1. what I did, was setting the prices of the items in my shop to 1000 and then adjusting the price ratio.
I hope that helped you. The shop your looking at is named "deco_effect" so look fot that inside the MarketResource