When a person donates an item to a charity for an auction the charity can give the donor a receipt for the eligible amount of the donation (fair market value minus any advantage the donor received).  When a person bids on an item in general they would not be able to receive any official donation receipt unless certain CRA requirements are met as noted by CRA below.

Summary Policy
Date
June 9, 2003
Reference Number
CSP – A15
Key Words
Auction – Receipt – Fair market value – Eligible amount of gift
Policy Statement
A registered charity can issue an official donation receipt to the donor for the fair market value of property donated for a charity auction.
Generally, a registered charity cannot issue an official donation receipt to the persons who buy items at a charity auction. However, where the value of an item can be determined and is made known to all bidders in advance and where the amount paid for the item exceeds the posted value, a registered charity can issue an official donation receipt for the eligible amount of the gift (i.e., where the posted value of the item does not exceed 80% of the accepted bid.)
References
• Donor, CSP – D17.
• Gift, CSP – G01.
• Receipt, CSP – R02.
• Income Tax Technical News, Issue 26.
• Registered Charities Newsletter, Issue No. 24.