[David L.]

"to be in business" is not in itself an idiom, but a grammatically correct phrase.
It is an idiom in the context, where something is broken and is mended, or put back together, or set up, and when it it operational, one person might say to the other, "Now we're in business!" The person is not, and does not mean, that he is ready to conduct business, or sell anything. It means that now the thing is working, or has been set up, they can get on and do something that was held up until it had been fixed or assembled.
An example: say I want to change the washer on a tap, but I don't have the right tool. If somebody says, oh, I've got a wrench in the boot of my car, and goes and gets it. When he hands me the tool, I might say, "Now we're in business" because I can get on and change the washer.