I haven't met anyone who based a buying decision on whether or not something was 'official'. But aside from myself, the other gamers I know don't spend a lot of time here interacting with the community at large or the publishers who hang out here. When their RPG itch needs to be scratched, they are going to gravitate to WotC material for two reasons.
First, it's more accessible, since they'll see it browsing the sci-fi section at the bookstore instead of having to make a trip to the NSFLGS. While there is some third-party material there too, they have pretty much the entire current Wizards catelog. Kind of a self-fullfilling prophecy in that the Wizards stuff is stocked because it sells better, but part of the reason it sells better is because they stock it. That's always the nature of the beast with the retail book world, though.
Secondly, while Wizards has put out some less than stellar material, their overall track-record is pretty damn good. For the casual gamer that doesn't know one third-party publisher from another, going off the beaten track can be a riskier proposition. While the d20 market has weeded out some of the slackers (and taken some good ones out too), the initial glut created some bad impressions. I think that was a price worth paying for the OGL, but it still kinda hurt.