this is a key example of poor customer service and one that really shits me. If you're a retailer selling a product in bulk, as opposed to an individual selling a single new or used item they have, and then wash your hands of it if there's a problem, well that's just shit now isn't it. In this age of faceless businesses operating online, being prepared to take accountability as the point of sale would be key to instilling confidence in your customer. Yes you may not be the manufacturer but if you're promoting something as being suitable for a certain application and it's not, then you need to apply more diligence to ensuring you're accurately marketing a product, and deal with the repercussions directly with the customer, as it's your business they won't be returning to.
(I have no idea on your personal business or the transactions referred to above, I have just been burnt before by companies and been out of pocket paying for a rubbish product...it's shit.)