This has been driving me mad for ages and I still haven't come up with a definitive answer.

The way I would use the two phrases are thus:

'On the market' - a product would be on the market or can be launched onto the market for sale.

'In the market' - a person or company would be in the market to buy or sell a commodity.

Does anybody agree or disagree with this? Is it simply a British / American usage thing? Any suggestions would be most appreciated!