1. They are at the market.
2. They are in the market.
(Which preposition is correct?)
3. Who is selling traditional food?
- Some women are selling traditional food.
4. Who are selling traditional food?
- Some women are selling traditional food.
(Which verb do we have to use, 'is' or 'are'?)
Both 'in' and 'at' are correct, but with slightly different meanings. Use 'in' when you mean the person is physically inside the store or marketplace. Use 'at' when you mean it rather generally, maybe a substitute for 'she has gone shopping.'
I'd use the verb 'is' for your question about selling food. The answer may come back in plural form, but sometimes you don't know that in advance. You'd use 'are' if you KNOW the answer will be in the plural.