can't buy/sell more than 120 items at a time.

Irrelevant.

A) Not a new issue- introduced as a feature to combat an exploit years ago
B) Behavior removed in new client

it's still in the new client.

At least i couldn't sell my 400 units all at once, but they were sold in batches of 120.

We still have a limit due to people breaking stuff when they bought thousands of addons all at once.

what's the limit? and is it possible to remove the limitation from sale of commodities without breaking stuff?

Can't buy more than 2 items at a time now. This is on new and old client.

For add-ons - there is a buying limit of 5 and a selling limit of 1 unless the add-ons are in a ship and you confirm the sale despite it not being empty.
There is no limit on buying or selling commodities unless you use the max button in relation to the cargo on your ship when purchasing or if you use the unload and sell button.

This is on the beta 7 Mac client.