Without knowing the site in question, and knowing that even table designs can validate, or that sites that don't validate can even do well -- or not, if the coding is so poor that not even SE's can make sense of it -- one may speculate a huge possibility is that the site isn't converting well because of a design that could use a marketing makeover.

If ads are on page one, and the site is getting traffic but no conversions, then it's a layout / marketing problem on the landing page. Or navigation-friendly issues of the site. Or too many pages that have to be viewed before a customer can check-out. Or a number of marketing fouls so that people just aren't buying. Personally, I would address that before investing in anything else.

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DesignerTrade