For those wondering about the structure of this sentence, here is an explanation:
Normally using inversion as a way of asking questions takes the form: conjugated verb + hyphen + subject pronoun (example: Parlez-vous français? - Do you speak French? )
However, when the subject is a noun phrase (i.e. not a pronoun), it may be inverted directly with the verb if:
• the question word is short, such as où, que, etc.
• the subject noun phrase does not clash with the object
the general construct is: conjugated verb + person
Où travaille ton père ? - Where does your father work ?
Que veut dire ce mot ? - What does this word mean ?
Que fait votre tante ? - What is your aunt making ?
Qu'ont bu les étudiants ? - What did the students drink?
OK, I know I should know this already, but I've got to ask - in the heat of battle as it were, in the middle of a conversation, how can one avoid thinking at first that the "que" is the subject of the verb "vend"? My gut instinct on first hearing this was to translate it to "what sells this shopkeeper". An absurd sentence, I admit, but change a word or two and it's OK. So, I just need to make sure: "Que" does not mean "what" at the beginning of a sentence, and "what sells this shopkeeper" would be "qu'est-ce qui vend ce commercant?"
It is a good word but it is not the translation of "commerçant". A "retailer" (EN) is "un rétaillant" (when talking about a company) or "un rétaillant, une rétaillante" when talking about a person. It is important to understand that "un commerçant" refers to a shopkeeper/storekeeper which one would generally understand as the proprietor of a small business (shop).
In UK english it would be more common to ask what a shop sells. Shopkeeper is a quite old fashioned expression and storekeeper is a US expression - if referring to the actual owner we would probably use shop owner above shopkeeper, or if a large organisation would use the term retailer. I believe that in France it would also be more common to ask what ‘le magasin’ sells.
While each of these uses might not mean exactly the same thing, "merchant" is used more than "vendor" and "shopkeeper"/"storekeeper" not much at all.
"vend" here is a conjugated form, 3rd person singular present tense, whereas "vendre" is the infinitive (cp "TO sell"). An infinitive can be the subject or an object of another conjugated verb. All French infinitives have an R as either the last or the second to last letter (here: vendRe).
The dictionary gives trader as one of the definitions of this word, so it should be accepted. Also, a storekeeper is not a shopkeeper! A storekeeper would be in charge of a warehouse, perhaps. I hope it will be replaced by shopkeeper in the sentence and trader should be accepted as an alternative.