The older I get, the more I come to realize that "ad blindness" infects other areas, some of them important, and people have become blind to nearly anything on a page except for exactly what they are looking for. This includes any of the "badges" of credibility/reliability, even SSL cert thingies. If I'm looking for widgets and the page contains anything but widgets, I'm blind to it.
This is not to say these are bad, and shouldn't be used, they should be present for what I feel is the minority looking for them. To answer "Indicators of credibility" - the one thing a large percentage of sites out there miss?
Make sure everything works, and works as expected, for everyone. (please.) Especially if you're selling items. You'd be surprised at how many site owners are blind to their own tunnel vision.
Descend into other items from there, second in line is an overall "make it real." Business contact info, brick and mortar even better, adhere to regular business hours ("Phone in orders only on M-W-F between 3 and 5" ugh.) You don't have to say you're a serious business, show it.