I always look for reviews of online sites - there are sites out there specifically for consumer reviews and you can rate / review anyone and/or anything. One bad review doesn't deter me, I look for consistency, and in particular, read between the lines to try and determine how the company dealt with the problem the customer is complaining about. 

I also check their Facebook page (if they have one), to see how active they are and what customers are saying there. That saved me from a poor purchase decision last night, actually, as I found a site that had several things I was interested in buying, but their Facebook page made it clear the risk wasn't worth it. 😉

At this point in time, I think introducing a transaction count could be beneficial, though I'm not sure a single global rating out of 10 would work in practice. If you had a transaction count (for sellers), then I suppose maybe you could have a simple "satisfaction rate" as well, not out of ten, just a yes or no/ eg 1000 transactions, 995 satisfied, with optional reviews on the feedback page. (A bit like the comments you can enable on My World, but can only be left by customers).