Warren Ellis’ forum The Engine has a thread on webcomics and the direct market (i.e. comic book shops). The discussion ends up being more about the problem retailers see with creators debuting their comics at cons without offering them at shops at the same time, but also has some interesting discussion for webcomics with an eye on the traditional comics market.
Retailer Brian Hibbs:
In most cases, my knee-jerk reaction to something (anything) that is being made available to me secondarily is going to be minimal if not nil orders. […]I’ve got no real concern about creators having an equal or better crack at the hardcopy sales, but where the advantage directly turns against me (ie: offering for sale BEFORE I have a fair crack at the work… be that on-line, or, yup, even in person at a convention or something), then I’m way way WAY less likely to support that work with my purchasing dollars as a retailer.
Hibbs elaborates a bit on this saying that he does not see as big a problem with comics offered free online. The problem is if the consumer has already paid for the comic in some format, which would make him/her less likely to buy it again through a store.