Interesting point you make, as I see the dilemma . His and him, as opposed to only the word her. As a native speaker I never think of these small quandries , haha. The way to say it is to put it in context, meaning you need to add additional words to clarify your meaning. So,, You say I will buy her flowers, usually without otherwise stating, means you will buy flowers for her. Since that is the common use. You can also be very specific and say I will buy flowers for her. If, you wanted to state the other meaning you would have to say, I will buy flowers from her. Need to use , from. Or say I will buy the flowers she is selling. --- Also, your sentence " "Buy flowers to him" " -- should be from him, not to him.

It does have 2 meanings, and there isn't a way to avoid confusion, you just have to know by context. Usually it is very obvious to a native which one is intended: "I met a cute girl last night. I'm going to buy her flowers." (for her) "My friend opened a garden shop, so I'm going to buy her flowers instead of the ones at Walmart." (flowers she is selling)

A native speaker would take the meaning from context, as usually only one makes sense. . Your wife is annoyed with you. This lady has nice flowers for sale. Buy her flowers. <-- ambiguous, but unlikely Your wife is annoyed with you. This lady has nice flowers for sale. Buy her some flowers. <-- her = wife as the seller likely has too many flowers and "buy her flowers" sort of means "all her flowers" Your wife is annoyed with you. This lady has nice flowers for sale. Buy some of her flowers. <-- her = seller

A grammatically way of saying "Buy him flowers", or "Buy her flowers" would be, "I buy him flowers." or "I buy her flowers" A pronoun "I" is needed. In either case there is only one meaning which is, that you are purchasing flowers (probably as a gift) for him or her. Hope this helps.