This could differ slightly depending on your product mix and category, but in general you can think of them as different tiers in terms of traffic. Amazon is Tier 1, Ebay and Google definitely are below that and are Tier 2, everything else is Tier 3. Obviously because Google Merchant (Google Product Search) is free it has the most bang for your buck, Amazon is difficult to integrate with unless you have a tool at your disposal so it may be worth investing time into that channel when you have a lot of it or a partner to do it for you, and for the other marketplaces, Buy.com and Sears are a little painful to integrate with as well, at least compared with a program like Google Product Search. I hear Ebay is pretty easy to integrate with & haven't heard much about Newegg's volume & integration process after it's drop off in the early 2000's.
I agree with CPC_Andrew in that it varies by niche. For our items:
eBay Amazon 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.