Both iphones you sold were sold as used.
Used: An item that has been used previously. The item may have some signs of cosmetic wear, but is fully operational and functions as intended. This item may be a floor model or store return that has been used. See the seller’s listing for full details and description of any imperfections
You should have sold them as - Parts, not working if you were unsure they were locked or not.
Under eBay's terms of used condition the buyer is quite within their rights for a refund.
