Yes, the commission charged by the auction house would be considered an expense of the sale. This would be similar to the transaction fee charged when a stock is sold.
As you enter the information into TurboTax, there will be a question about whether you had expenses for the sale. The commission is entered in this section and it is added to the cost basis of the item that was sold.
See the following TurboTax help article for more information:
Where do I enter a capital gain or loss?
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