Imo the 'buy amount' field could use a validation preventing entry of non-numerical into it, it silently fails if you've done something like hit 'i' to check invent and appended it to your amount. Somewhat caused by user error but... yeah
Ideally the UI could pick up that you're actually trying to open inventory instead of buy 50i freon foundation.

Also buying > 50 of an item when a blueprint needs 500 is a pain in the ass
eg. project big chill, freon foundation.