Unfortunately there is not much of a market for used NFA weapons (except machine guns obviously, and sometimes more difficult to source DDs, but those are not at issue here).
If you sell to someone in your state of residence, you can do one Form 4 from you to him. This requires the same wait and tax as any other form 4, there's just no dealer involved.
This is usually pretty difficult to do for people who aren't otherwise friends because it involves a great deal of trust. He will have to give you the money up front (or should anyway) and then he won't see anything for maybe 6 months. And as the seller, you will be receiving the completed transfer form. So if he doesn't trust you to call him and let him know that the transfer has been approved it could turn into a mess. And not saying that you would, but there is the risk of the seller just canceling the transfer and walking. Most of the risk is on the buyer and not the seller, but that means there are fewer people who will want to buy from you.
(As an aside to an already long post, I had to give up on an awesome price for a MAC 11/9 from a friend's estate because I couldn't trust his widow to hold up her end of the deal)
The only way I'd do this would be if it was a transfer between friends OR (in the case of a machine gun or high dollar weapon) sufficient safeguards were put in place like contracts, escrow agencies, lawyers, etc. I don't know about you, but I wouldn't think it's worth it to go through all of that for a form 1 SBR.
If you sell to someone out of state, you'll very likely have a 2 stamp (per item!) process. Because the gun has to go through a dealer in the buyers state, the process will be: Form 4 to a dealer in your state ($200 Tax paid), form 3 from dealer in your state to dealer in buyers state, form 4 from dealer in buyer's state to buyer ($200 tax paid). There may be a way for you to do a form 4 from you directly to the dealer in the buyer's state, but I'm not positive. Someone else will need to answer that question). So in effect, you would have $400 tax due per item to sell the NFA weapon to someone out of state.
Option 3 is to sell to a dealer in your state (which is probably the easiest route for you, but will net the least cash.
Now you're starting to see why a lot of people consider NFA weapons "lifetime purchases" :banghead: because of all the taxes and hassle, people would want a significant cost savings over building their own. Because sellers really can't make much money, there's not much of a secondary market. That said, most of the suppressors I have were "used" (dealers rotating out their lightly used demo cans). Most of the used cans I see are $200-400 for decent 9mm cans, and honestly anything more than that and I'd just buy new.
So, after all of that, I'd reconsider if you really wanted to sell, but if you do I would recommend selling the gun as a title 1 weapon (i.e. No longer a SBR). And figure out your absolute bottom price for the can and see if anyone you know wants it, or if you could sell it to a dealer. If you do decide to sell any NFA weapons, be prepared for a bunch of people to try to get you to make illegal transfers (i.e. "I'll give you $200 more if we don't worry about the paperwork" type of thing).
Good luck!