A fully W3C compliant website is a boring website. Especially true on the large scale.
The accessibility = sales thing is complete PC tosh. I'm not a guy for 'target markets' and conversion rates etc, I think it simply detracts from the main issue, that your site is crap.
However. Of you target market, what percentage of them would require accessibility? Now what percentage won't buy from you because you have a completely uninteresting website?
If the developers spend all their time going after standards compliance and accessibility rather than building in features the majority of the userbase desire then it won't matter how much you comply with the law to increase conversions, you'll get neither userbase.
How accessible do you think Amazon is? It's not, in the slightest. You know what they do for people with screen readers and such devices? They stick 'em on the mobile site. Or for the visually impaired they push you to a site that tries to sell you braille books and voice recog. software. None of the font is bigger, none of the images are larger, nothing has changed, they just now know what trash they can sell you.
I'm Not Kidding, Take a look
It may be discriminatory but unless you are in the business of selling glasses, screen readers, hearing aids etc etc etc then it simply won't do you any good but win you brownie points with people who make it their business to make up awards for those who pander.
It's not just Amazon, pick any big online retailer you like. None of them give it a second look. The effort wasted going after such a small market is better and more efficiently spent going after other areas. Until there is some kind of proper monetary incentive (like a fine) it will always be that way.
It's the same for W3C standards. So long as it works for the majority of browsers in the same fashion, why bother caring about W3C? Some browsers don't even comply to WC3. Build for the customer, not the regulations.