At this point, you can buy today and sell tomorrow and make money if that's your game.
Tell me where my logic is flawed here.
You would need buy under market rate (not that common right now) so there is zero percent chance of making money the next day. In most places you won't even take ownership in one day, let alone through guns.com.
1 hour buy, 1 hour sell, 1 hour unpacking/packing, 1 hour driving.... lets say that is $200 of your spare time and I betcha a "good" gun would take more time.
If you are not in state or an FFL, then add $50. If you pay shipping one way add another $50. Straight out of the gate this resale needs to net you $300 just to break even.
You are going to need to buy at $900 and sell at $1200 the next day to break even. To make 30% margin, you are going to buy at $900 and sell at $1470.
Good luck with that, especially with larger firearms that are soon to be a minimum of $200 each per year just to own.