Only under specific circumstances.
Charitable donation receipts can be issued only for gifts. They cannot be issued for purchases of goods or services. Your auction patrons have bought something; that’s different from an outright gift. (Here’s the citation from the CRA website).
In the case where an auction patron has paid more than the fair market value of the item, a charitable donation receipt can be issued for the portion of the auction price in excess of fair market value if these three conditions are fulfilled:
- It must be possible to determine the item’s fair market value
- The value must be posted before the start of the auction
- The fair market value cannot exceed 80% of the purchase price
Here are two references to CRA articles that discuss receipting:
To restate point #3 in a different but equivalent way, the buyer must pay 25% (or more) over the fair market value of the item.
Let’s say one of your auction items is a camera with a fair market value of $400. In order to qualify for a charitable donation receipt, the successful bidder must pay at least $500. If you do the math, you’ll find the $400 fair market value is 80% of $500. A $500 successful bid is 25% over the $400 FMV.
If the successful bidder paid you $500 for the camera, they would be entitled to a charitable donation receipt for $100, the amount in excess of fair market value.
What if the bidder paid more than $400 but not quite as much as $500? Well, if they fail to meet the test, then they’ve just bought themselves a very expensive camera! In the CRA’s eyes, that 80% rule is the threshold for establishing that a donation has been made.