The meaning is slightly different.

我想在超市买水果 means you "want to go to the supermarket" and the reason is that you want to buy fruit.

"我在超市想买水果“ implies that you are already at the supermarket and would like to buy fruit.

That's what I put, too. It should be included in the answers.

You put the abverd last in your sentence, which grammatically is wrong

Exactly. I too want to know

Not exactly… 到 means "arrive", it's a different meaning than 在. Although you may want to arrive at the supermarket before buying the fruit there, the sentence is more specific and describe where you want to buy the fruit, not where do you want to go (in order to buy fruit).

Hello there! both 想 and 要 mean "like" or "want" and by putting both of them together it becomes redundant. I hope this helps