In agreement with talrnu’s comment, I think “window shopper” is in fact appropriate here. In usage (which might be better represented on Urban Dictionary and Yahoo Answers), this term is commonly employed to refer to people who walk around both inside and outside of shops, and is used to differentiate such people from those who have more of an inclination to make a purchase.
From Urban Dictionary:
Someone who looks at stuff they can’t buy.
When one visits a store or mall to admire goods rather than to purchase them.
From Yahoo Answers:
It implies [. . .] someone who is not serious (a shopper who has no real intention of buying [. . .])
“Browser” does not sound right to my ear as it seems like an awkward nounification. Most people will think of something they use to view websites. I have not heard the word used this way. It would probably require explanation, which would defeat the purpose of having a single word for it.
That said, I have heard people say that they are “just browsing” to indicate that they are not actively pursuing a purchase at that moment. For your case, I could certainly see someone saying “he’s just browsing.” As far as saying “he’s not a customer”, you have another problem.
Anyone visiting the store is, potentially speaking, a customer. so it’s not ideal to say “he’s not a customer” unless you are pointing to an employee, security guard, pomeranian, etc. You might be clearer if you qualify the word “customer” before you establish this comparison, for example: “he’s not a serious customer, he’s _____________.”
Lastly, another term I know for such a person is to describe them as a looky-loo.
looky-loo
(also lookie-loo)
NOUN
INFORMAL
1.1 A person who seems interested in making a purchase, but whose actual intention is only to browse:
a treat for all the North Shore’s looky-loos: the popular Spring Designer Kitchen Tour
A good business broker will separate the real buyers from the looky-loos, bring in more qualified prospects, and usually can garner a better price for the business.
Source: Definition of looky-loo in Oxford Dictionaries
Colloquially, you can be a fan (or not a fan) of anything, abstract or concrete, tangible or intangible.
When it's used colloquially, be a fan of implies "like".
I'm a fan of Tom Cruise. ~ The speaker is a fan of the actor. They surely like the actor.
I'm not a fan of spicy food. ~ The speaker doesn't like spicy food much.
Here is a good example showing that the expression "a fan of" can be used with virtually anything. It's from the movie Edge of Tomorrow (the quote is from www.moviequotesandmore.com):
[as they drive along the main road in France]
Cage: You don’t talk much.
Rita: Not a fan.
Cage: Of talking?
Rita: Not a fan of talking, no.
It just means that Rita doesn't want to talk (with Cage) in that scene.
So, your I am not a fan of negativity simply means: I don't like negativity.
Best Answer
No is a quantifier, like one, some, many, much, few, three. We don't combine quantifiers except in mathematical expressions (one hundred, three dozen)†, so no many is illegitimate.
Not is a negator. It can be combined with quantifiers (not much, not one) and other word classes (not difficult, not quickly, not a question). Generally, however, we prefer to 'raise' the negator in a predicate and attach it to the verb, just as we prefer to 'raise' it out of a subordinate clause into a main clause. The unraised versions tend to be employed only to emphasize where exactly the negation lies.
† You will see expressions like No three of them could lift the rock. Here no doesn't combine with three, it quantifies the entire following NP; that is, this sentence is equivalent to "No set of three of them could lift the rock".