Merchant Frame Extended is a small addon that lets you extend the default 5 rows, 2 columns merchant frame up to 10 rows and 6 columns. The direction in which the items fill the window is also adjustable.
Now also mostly available in German and French!
More features can be requested via the issues system or discord.
Feel free to join discord and say hi: https://discord.gg/mdBFQJYeQZ.
Creating and maintaining addons takes a lot of time. If you can miss something, feel free to drop a donation!