It's a bit more risk to sell on Ebay and depending on the value of the bag, you may get a higher price for it via a reputable consignment store which people trust and follow.
Go to Ebay and check the "sold" listings and see if you can find a similar Gucci Marmont and see how much it sold for (ignore the listings for obviously fake bags which sell for minimal). Then have a look at consignment stores such as
Fashionphile or Yoogi and see how much they're selling for. You may be better off selling to the consignment store even if they take their cut. Both will give you a quote just by sending them photos so you can see if you're happy with the quote. It will be a lot less headache.
If you want to go ahead and sell on Ebay then I'd agree to get it authenticated, there's plenty of online services. Usually authentication via photos is pretty cheap and has a quick turn around. You will definitely get a higher price for it if people looking through Ebay can differentiate your listing from the gamut of fake bags on there.
List it for the price you're happy with and if someone sends you an offer (if you allow offers on your listing), check to see if they have good feedback or if they have zero feedback and have sent you a suspiciously high offer. Remember to not send the bag until you have concrete payment in your paypal/bank account (sometimes people will send you a "proof" they have sent money, but if it hasn't appeared on your end, don't send your bag). Film your boxing it up and get a photo of the postage label indicating the address its going to and the tracking number IN THE ONE PHOTO. Make sure you send it requiring a signature on delivery (so the postman doesn't just drop it on their doorstep and they can claim they never got it).
This should eliminate most issues but there can be still others depending on how savvy your buyer is. I don't want to scare you since I and many people have sold successfully on Ebay before with no issues. But there are definitely risks there as BeenBurned says.