2.are street markets common in your country?
Yes they are. well I guess, the reason is that Street markets have been historically existing in china since the tang dynasty. I think It’s part of Chinese culture. Besides, People can purchase basic necessities at a low price in a street market. So folks are quite keen on it. In general, it happens either in a village or in a town.  
3.what is usually sold on markets?
As i said earlier, there’s many organic vegetables which are good for our heath. It’s fresh and cheap. So everyone can afford it. What’s more,if you were there, you will smell the scent of food. We call it “street food” which provides residents an opportunity to get together,talk and discuss. Like”oh this is my favourite store”” so am i . that’s my favourite one”. everyone can enjoy in. Above all, you can buy clothes,toys, kitchen utensils and etc. cái này nghe có tự nhiên không?

First word of the questions should be capitalised. The clause "Well I guess,..." is most commonly used when you are unsure of the answer, but you have already stated plainly "Yes they are" in the previous sentence. "People" shouldn't be capitalised as it's in the middle of a sentence and is not a 'Proper Noun'. No full stop is required between "market" and "so"; it can be a single sentence.

You use the plural "organic vegetables", then in the next sentence, you use the pronoun "It's", which is singular. It should be "They are fresh and cheap". Similarly "can afford them" in the next sentence.

Double quote marks after "favourite store", whereas only one " is needed. When replying to "oh this is my faviourite store", someone would say "mine too" rather than "so am i".

"Everyone can enjoy it".

"Above all" is used to make the strongest point or most important point of an arguement or description but here it seems "Also" would be more appropriate.