The
When we use the definite article it does not mean that the thing spoken about is unique, ie that only one exists in the entire world. It only has to be unique within the context - your audience has to know which one you are referring to.
So, if you're looking at this range of toy cars and you say "the green car", your audience should realise that you mean the green car from this range, because there are lots of green cars in the world but by saying the green car you must mean a specific one. It's an unwritten rule that, when you refer to something specific, the audience should either know what/who you are talking about or it should be immediately apparent.
If you said "we don't want to stock green cars" that would mean you don't want to stock any green cars as a rule, and that you would not carry them from any range, which actually doesn't sound so unusual - for most of my life cars were rarely produced in green due to superstition!
The difference between "we don't stock..." and "we don't want to stock..." is that the latter expresses your desire. It sounds less idiomatic in the context because it sounds like a personal preference of the shopkeeper rather than a business decision.
That
'That' as a determiner is used to refer to something specific that has already been identified. I know you've talked about swimsuits, but let's stick with the green car for continuity. Someone comes into your shop and asks for the green car, you could say that you didn't stock "the green one", or you could say "that one". When you use 'that', you don't have to mention the colour again because the person just mentioned it - it has already been identified.