Where can you go to safely sell a used Remington Model 1100 12 GA shotgun?

Answer 1

You must understand your state laws if you sell it locally without a FFL involved, you didn't mention the state so I/we can't help you with that.

The only other SAFE way to sell it online, locally or whatever, is to require whoever wants to purchase it to receive it through a FFL. (Federal Firearms License holder) This is what is done with all online sales. (unless it's antique or C&R which this is not)

In short what this means is the person who buys it must provided you with a signed (or now recently law changed faxed) copy of the FFL and you MUST ship to the address on the FFL form and put a copy of the form in with the firearm when shipped in case it is inspected.

The ATF has an online form to verify the authenticity of the FFL.
https://www.atfonline.gov/fflezcheck/

Basically if you ship to a FFL which is validated on the page (along with their address) you are complete legal. That Federal Firearms License holder is obligated to follow all federal and state laws for the transfer of the firearm to the end user.

Of course all gun shops are FFL holders so you can just walk in a sell it to them but be prepared for a STEEP dealer discounted price for it.
Browse gunbroker.com, gunsamerca.com, auctionarms.com you can see lots of people selling online and it's a good place to get a comparable for the asking price.