I've bought one bike each from eBay and Craigslist.
Craigslist gives me a better local purchasing option than eBay due to the sheer quantity of local bikes available. Which means I can see the bike in person and even take a test ride before purchasing.
It seems like all the bikes I am interested to buy on eBay have to be shipped, which is costly and the bike can be damaged in shipment. Plus I don't get to see the bike in person on eBay. I can only go by photos and there may be mechanical issues that aren't disclosed.
While I like eBay over Craigslist for many things, I like Craigslist over eBay for bicycles.