Can anyone tell me how profitable it can be trading the discrepancies between the different ECN prices during the pre and post market hours?
I don't think their are many... most times I have seen what looks like a discrepancy it is a stuck or bad quote . Just my 2 cents. monee
Many times I have used the pre market quotes successfully to short a stock as the up tick rule does not apply.
To add, how often is there a discrepancy between ISLD and INCA bids and offers during the pre and post market allowing one to exploit these price differences?
I think that most (or all) of the discrepancies you will see will be from MMs not ECNs, and the MMs won't honor their quotes (you can always try to hit the MM first and see though). When trading pairs, I will enter pre-post, and it often works out. I'm not sure about entering a single position though.