I would have thought, unless you were a botanist talking about seedpods, or wanted to talk about seafood ("frutos do mar") or the fruits of your labours, "fruta(s)" would be the most common choice. (I just checked the dictionary and surprisingly "fruto" is a synonym of "fruta": http://www.aulete.com.br/fruto).
Fruto is the technical name in Biology for what plants (angiosperms) produce in order to generate a new plant (of course there is a technical definition for it, but I believe it's not important here). So, by this definition, a tomato is "um fruto", as well as a nut.
Fruto is also the abstract concept of fruit, the result of something.
Now "fruta" is a popular word for juicy and generally sweet fruit one can eat. (So, tomato and nut don't fit here)
'Fruit' is commonly used as a collective noun in English. As a native English speaker I would never say: He sells fruits
You could say it if you were referring to multiple varieties of fruits:
"He sells fruits (apples, oranges, and bananas)"
In the previous question that I just did, it marked "fruto" as wrong and said it had to be "fruta."
Please, can someone give me the correct way of saying this? Are they truly interchangeable, or is there some situations where one would be more correct than the other?
I think if you mean edible fruit then "fruta" is the best choice:
Probably because it originated from the English sentence and both "frutos" and "frutas" were used in its translation. Less likely, it could be that the implication is that the seller sells more than just juicy edible fruit.
Try a Google image search of the .br domain (site:br) for the quoted string "Mercado de frutas" (lots of photos of stores full of lovely fruit) and "Mercado de frutos" (lots of photos of artisanal goods outlets - fruits of their labour - or seafood - fruits of the sea - and some stores with fruit and nuts).
Because this sentence is wrong.. I just reported it.
We would never say "ele vende frutos". For seafood, we say frutos do mar.
I wrote "Ele vende frutas" It was marked wrong. I don't understand why. I tried to report it but the report option doesn't allow me to :-(
After I got married someone jokkingly said to me "cade os frutos?" Meaning where's the fruits of my marriage (aka children). So frutos has a meaning similiar to phrases like "the fruits of your labor" etc.