I'm not sure why you're asking this question in the Online Bidding section, as that might be treading in dangerous territory.
You must hold a Federal Firearms License (FFL) to take firearms on consignment. If you had the license, you would most likely be aware of all the required forms and record-keeping requirements.
The only exception to the FFL requirement would be in the case of selling firearms for a single individual, such as in an estate auction. In such cases, there are no specific forms requirements although it's a good idea to get the buyer's drivers license info. Also, you would want to make sure that you are not selling them to someone in another state.
This topic has been cover pretty well in the past. I would suggest doing a search in this forum for more specific information relating to selling firearms at auction.
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