Something that is in the market exists physically within some actual location where they can be bought.
I saw these same shirts in/at the market over on Front Street for half the price.
On the market is more of an idiomatic expression that simply indicates something is available for sale.
These new dresses have been on the market for only two weeks, and most stores are already sold out.
Either can work for what you want to say. If you saw the monitors within some specific, physical market then you would use in (or at). If you are talking in general about the ability to buy the monitors, then on is more appropriate.
That being said: In most contexts I wouldn't use either. I would use the name of the place where they can be bought:
You can buy these monitors from Amazon.
WalMart has these monitors at a good price, if you want to get a couple.