It depends what the items were.
You can issue tax receipts for gifts of property. Thus, the donors of tangible auction items – artwork, an iPad, a bicycle, anything you can touch – can receive a tax receipt for their gift.
You cannot issue tax receipts for gifts of services (e.g. a tax consultation, a spa treatment) or for promises of services (e.g. a gift certificate, a voucher for hotel accommodation).
One important point of clarification: the company that issues its gift certificate or the hotel that issues the accommodation voucher has given you only a promise; they have not transferred property. However, if someone other than the issuer purchases a gift certificate or voucher (i.e. pays for it with cash) and then donates it to you, they have acquired property and made a gift of that property – and in that circumstance, a tax receipt can be issued.