[Grammar] I might/could say one of these sentences

[Tarheel]

Does the addition of "I don't know" change anything? Would you make the same comments as 1) and 2) above?

A: Where is John?
B: I don't know. He might be in the market.
B: I don't know. He could be in the market.

Possibly:

Abe: Where is John?
Bob: I don't know. He said he was going to the market.

(I realize that's not what you're asking about, but it's a real world possibility.)

 

[Charlie Bernstein]

But analytically, "might" is "real" and "could" is "unreal" still. Am I right?

Maybe sometimes. In your examples, He could be and He might be seek the same information and would get the same answer:

Me: Where is he?
You: I don't know. He could be in the market.
Me: Okay! Then let's go to the market and see if he's there.

Me: Where is he?
You: I don't know. He might be in the market.
Me: Okay! Then let's go to the market and see if he's there.​

Either way, it's possible that he's there and possible that he's not. So for conversation, it's not a matter of real versus unreal.

A note on usage: It looks like you're using in the market to mean shopping. Usually, at the market means shopping, and in the market for means seeking to buy:

Me: Where is he?
You: He's at the market.

Me: Did his car die?
You: Yes. So now he's in the market for a decent used car.​