I highly suspect that if you tell the retailer you bought the power supply from that "the motherboard I bought somewhere else just killed the power supply you sold me", they will not be sending a warranty replacement, and will probably laugh.
I also suspect that if you tell the retailer that sold you the bad motherboard that "the motherboard you sold me killed the power supply that I bought somewhere else", their reply will be something like "how do you know that it wasn't the other guy's bad power supply, that killed that perfectly good motherboard we sold you? (actually, they will probably state it in the definitive - i.e. The power supply killed that motherboard, therefore not only will we not send you a new power supply, we won't send you a new motherboard either).
Moral of this story? Choose your words carefully when requesting warranty replacement on either product. One thing is certain. One of those devices killed the other (very likely motherboard killed the PSUs, but you may never know 100% for sure which), it probably wasn't your fault, but rather was the fault of a legitimately defective product that should be covered under warranty, and someone is going to eat it. Is it going to be you? Do you feel this was actually your fault? If so, then eat it, and learn. If not, choose your words carefully. There is no need to lie about anything or provide any false information (that would be wrong), just describe the symptoms truthfully and concisely, and request a replacement. Too much information can be a bad thing when a retailer or manufacturer is looking for a reason not to honor a warranty (and they usually are, as honoring warranties costs money).