The auctions being "public", they are open to anyone who can bid, i.e to any creditworthy person.
The place where the sale takes place is public. The auctioneer ensures the free access to the sale.
The restrictions to participate in an auction are those of common law : non-emancipated minors, protected adults, people under the influence of alcohol.
The auctioneer may subject the participation to a prior registration and the presentation of a guarantee of payment, these conditions being objective, non-discriminatory and appropriate.