I purchased this at auction as it was advertised as a 20 gauge and that's what I'm gravitating to now. Turned out to actually be a 16 gauge and I'm trying to find best way to market it for sale and fair price. Anyone have any recommendations. We're in Middle TN.

Welcome to the forum, Tom. This is a good place to have lots of exposure to the vintage doublegun "community". You should put the advertisement in the Doubleguns For Sale sub-forum here, along with all of the details on the gun that you can garner. The more info you can provide the easier it will be for potential buyers to discern what you have. Chamber length, barrel length, bore diameter, choke constrictions, length of pull, drop at comb and drop at heel, butt treatment, whether fluid or pattern welded barrels, and minimum barrel wall thickness are among the things most knowledgeable potential buyers will want to know. Then, good close up pictures should be posted here. if you need some help with that some of us here will be glad to assist you.

SRH

Call the auction company. I had this happen twice. Both times the auction companies offered to refund for return or keep the gun at a discount price.

Auctions are usually as is but when an item is inadvertently listed with the wrong gauge they usually step up and make it good.

Probably not a good idea for this gun, but I have chamber sleeved a 16ga to 20ga, and am happy with the result.
I bought a Elsie 20 at auction that turned out to be 16 and they kept it.
If you cant return it.....

With Lefevers, the 16 gauges are often lighter than the 20 gauges of the same model. You might like the 16 better between the two.

I have to second Daryl and Stanton's comments. Take some good pics, compile a good description and post it here, in the fore sale section! You have a good and possibly willing audience!

Chief