I agree with CPC_Andrew in that it varies by niche. For our items:
New Egg - seems to come in spurts.
Buy.Com - one or two orders a week
We are getting ready to launch on Sears. For the markets I listed above, we have spent money to automate inventory synch and are getting ready to automate the order process. If you have significant inventory you pretty much have to automate it otherwise you are cancelling orders all of the time. We also sell on what I would call 4th tier (Ebid, Bonanza, etc) but have zero orders so far. We did eCrater and had 1 sale in a month but we had to stop since they have no way to update inventory other than manually - too hard to keep up but it has a big following. Atomic Mall is out there, Blue Jay, Webstore, and others, but I am not hearing much success for these others.