"You can buy whichever you like, but not both."
Why would the sentence "Vous pouver acheter celle que vous aimer, mas pas les deux" be wrong? If I wanna buy 2 vases? §
You seem to have forgotten the correct conjugation of "vous" which has a -z ending for all verbs of the 1st group (infinitives in -er).
"Vous pouvez acheter celle que vous aimez, mais pas les deux" is correct, but not about two vases, since this noun in masculine.
Was it actually "acheter" in the answer or did I just put in this mistake when I was rewriting the phrase. For the noun, right, I thought about the German "die Vase", instead of French le vase.
Only the first item would be in the feminine form: vous pouvez acheter celle que vous aimez, mais pas les deux (deux is invariable)
How would you translate my suggestion: Vous pouvez acheter n'importe quel, mais pas les deux.
it is not correct because you used adjective "quel" without a noun, so you have to use the pronoun instead.
vous pouvez acheter n'importe LEQUEL* mais pas les deux