Hundred is a singular noun when quantified. So, it's "a few hundred". - A few hundred dollars. - A few hundred pounds. - A few hundred cats. It's the same as when you use an actual number rather than few. It's hundred, not hundreds. - Five hundred dollars (not five hundreds). The only difference is that you are giving an exact number rather than an estimate. Use hundreds if you are not quantifying it. - I've got hundreds of dollars. - There's hundreds of people here And hundred when you do: - I've got a few hundred dollars. - There are a few hundred people here.

I don't have a grammatical explanation, but I would leave out the "s" and say "I sold it for a few hundred." If you were to say "I sold it for a few hundreds," the meaning would be the same, but it invokes the image of physical hundred dollar bills. A ten, a twenty, or a hundred are used as short-form for "a ten dollar bill," "a twenty dollar bill" etc. So, "a few hundreds" to me sounds to me like "a few hundred dollar bills." The meaning is pretty much entirely the same though.

Hundreds

Normally in casual conversation you can leave out the S. But only in the sentence you gave. It is a bit like slang.