Who do you sell to the most? - This means "selling" in general.
Who do you sell it to the most? This means selling something specific, as "it" has to refer to something that was previously spoken of in the conversation.
Some people would insist that you use "whom" and one of the following, with the second being most preferable so as to not end with a preposition.
Whom do you sell to the most?
To whom to do you sell the most?
While some people will tell you that you should use "whom" in these sentences instead of "who", it's not at all necessary. Native speakers of US English almost always disregard the distinction between "who" and "whom" without even giving it a thought. Sometimes people use "whom" incorrectly in order to ensure that their language is "correct" or "proper". So in these cases, some people don't know why they use "whom". They don't understand the grammatical logic to which they are attempting to conform.
The answer is this: They sell it to people who play golf. They sell it to them.
If we replace "people who play golf" with "them", we find that "them" is an object pronoun, hence the use of "whom". However, hardly anyone abides by this rule in everyday spoken English in the USA. To me, it's a kind of "non-rule" because hardlyanyone observes it and those who try to observe it still get it wrong from time to time. I was going to save emails in which educated and well-spoken people use "whom" incorrectly. But why trouble myself? I know exactly what I'm talking about.
In formal types of discourse, we are likely to hear or read whom: legal and business documents, prepared political speeches.
USAGE NOTE The traditional rules for choosing between who and whom are relatively simple but not always easy to apply. Read more here: who: Definition from Answers.com