How much does it usually cost to sell an NFA weapon on the buyer side? What happens if the buyer fails the background check or never finishes it once I transfer the item to an NFA dealer to be transfered to a new buyer? Where is the best place to list an sbr to sell it?

Removing from SBR status is simplest way to sell an SBR.

In state all second easiest way.

If you are sewing out of state, there will be multiple tax stamp issues, outside FFL/SOT, long time lags.

As to failing a background check, you need to draw a contract indicating that if they failed the background check all money is forfeited, with the item returns to you.

Unless it is a high-end SBR, reverting back the title one is simplest thing to do.

Transferring to SOT would be the $200 form 4 stamp and wait time (not including the $200 stamp and wait time for the intended buyer). Imo, best to remove from registry and sell as a plain jane lower. With faster wait times on F1's, it's not worth the time alone to wait for even one F4 transfer to the dealer, then add in another F4 wait for dealer to buyer -assuming it's not some unicorn unobtainium lower

That's quite a deal if a seller can find someone stupid enough to sign such a contract. I'm pretty sure it isn't enforceable either, due to the legal theory of having ones cake and eating it too.  

Once sold, the seller has no legal claim to the item and the seller has zero ownership interest in that firearm. Whether the buyer fails the background check isn't really his concern.

If the buyer fails the BG check, the disposition of that firearm is up to the dealer. The buyer retains his ownership interest, but cannot take possession. Whether its NFA or Title I its the same.

True if the SBR was a Form 1 build, but doing that to a factory SBR lessens its value.

+1

Once the form 4 clears to the dealer, the new owner owns it. Whether they ever can get it transferred from the dealer or if they sell it straight from the dealer to another, not your problem.

In an interstate transfer of a NFA item with you as the seller, your involvement ends when you deliver the item to the dealer of their choosing (barring an agreed upon inspection period or anything)