Can we say, "This product has established itself on the market", meaning it already has its buyers etc. Is "on" the correct preposition?
I think it sounds strange, Miche.

'On the market' means 'available for purchase by the consumer': 'iPods have been on the market for some time now'.

'In the market' refers to the area of merchandizing for the product type: 'IPod has a high profile in the portable music player market'.

So I would go for 'has established itself in the market'.

By the way, did you notice that I had difficulty deciding whether to capitalize 'iPod' at the beginning of a sentence? It is a proper noun which begins with a lower-case letter, and I really don't know how to deal with it! (I prefer my first solution better, I think-- at least visually.)
Thank you, MM! My dictionary gives only "to be in the market for sth.", i.e. to look for some specific commodity to buy. Which does not fit my idea... So your answer helped me a lot.