Based on some real world incidents I know about, you haven't got much. The seller won't ship the gun until you pay for it (like any other on-line transaction) and once you do that you are SOOL if things go wrong.
Your FFL can inspect the goods for damages in shipping but should not, and probably will not, go beyond that. He is a transfer point; he is not responsible for the goods being as advertised or for making sure that you get what you paid for, nor does he have any responsibility for negotiating with the seller on your behalf.
In theory, if the goods are not correct or as advertised, you can return them and demand a refund. If the seller is honest, he will return your money, including the S&H costs you paid. If he ignores you or flat tells you to go jump in the lake, there is little realistically you can do. You can sue him in his state, which will most likely cost much more than the value of the goods. You can report him to the Better Business Bureau (he won't care because few buyers even check BBB records), you can report him to the authorities in his state, but small claims rarely get attention. One thing that will get you nowhere, is complaining to BATFE. They will respond with a polite letter, but they don't take customer complaints and they can and will do nothing.
Your best hope is to try and deal with a recognized business, not an individual, that has a good track record. Another thing you can do if you are stung is to publicize the case, but stick strictly to provable facts; if you stray into name-calling or baseless charges, he can sue you for libel in your state and it could well stick.
Jim