How much do you trust customer reviews that are on a particular retailer's website?

(...as opposed to on a general reviews site) It seems that retailers could simply use false usernames and reviews to “steer” a product to designated target markets (why else would retailers post those conspicuously poorly-reviewed products?), and in the same way, simply remove reviews which are not amenable to the underlying marketing strategy for a particular product. Has anyone else considered this? Maybe this is a known strategy in the online retailer network that I’m not aware of. Thoughts?