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)
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).