Does using flat textures that come with texture items (when bought from market) considered stealing?

I don't think so, but then again, I don't know anything.

I think it is stealing. You only bought the item not the rights for the flat texture

if the seller says that you can you use it than it's not stealing.
otherwise it is.

You don't own the art unless the owner of the art says you can have them. Keep in mind that the person that put the texture on the market isn't always the owner of the art that came with the item

often more times than not people buy and sell items without looking to see if there is even anything on it so most likely by the time you get the item to wear the maker would be long gone unless you tracked the history and asked every single person who had the item.

Well before he puts it in sale, he should replace the texture with a dummy texture.

I am not sure maybe buy im not sure

^ That.

If they sell a texture item with a texture image on it then by rights you have bought both.
Just keep in mind that if they do complain you'll have to provide proof you bought it with the image.

How can you prove that it game with the item.

You can always change the picture when you buy it.