On the market usually means that something is for sale as an individual item, ie a house.
My house is on the market.
There are a lot of houses on the market at the moment.
In the market, to me, means that many things are available within the same type of market.
There are lots of different designs in the laptop market at the moment.
As the previous poster said, we also say "I'm in the market for..." when we are thinking about buying something.