"Mercado" in Spanish is indoor or outdoor markets, it's not at all a supermarket. It has several sellers, not only one.
Indoor (the same, but with a wind/rain protective roof:
It is a public place (unless you talk about the economic market): https://en.wikipedia.org/wiki/Mercado
It's more distinctive that the English market/supermarket.
I think European countries have more outdoor markets than the English speaking ones (or just the US?), it's a tradition. In my city (in France), we have them twice a week), and we have one in almost all the cities or towns.
They give the translation "grocery store" for "supermarket" here, so report "grocery store/grocery" if it's not accepted.
In my country, grocery stores are very different from supermarkets. Grocery stores are small, opened later in the evening (and often expensive) and supermarket are big, and we find more kind of products. It's really bigger, none can make the confusion. But in America, I don't know why there's no distinction between the 2 kinds of shops?
Hey .. I have found out why:
"En el supermercado" don't work. It's not that you been spelling it wrong entirely. It's all in the "el" pronoun. In spanish when you type "el" as a lower case you're using this as if you're saying "He"
To differentiate that you'd have to change the "el" from "Él".
"el" = pronoun He. "Él" = pronoun The.
Because "ella" is She in Spanish. And "el" is for He.
So just keep in mind that:
"el" = He "Él = The in the Spanish language.
Because in Spanish they don't have a different words for THE by itself like in English does.
So what they do is: they just spelled it differently with a capital & an accent on top of it as that. If It's still wrong..let me know.