No lawyer or publisher here, but now that you know that it is clearly an illegal infringement of WOTC property, should it still be on your site for sale?
No you shouldn't police content, but once you know it is a violation of the OGL....aren't you complicent (sp?) in their stealing? Indeed, now that you know that it is a violation, I'd think you would get it off of your sight ASAP.
As a customer, I don't usually frequent stores that knowingly help others steal, and you may lose me if you leave a product on line that is in such clear violation.
Or am I missing something?