Best Case Scenario (You sell your 2070 a few days before 17th of sept, and you get a purchase for a 3080: Delivery lets say a week so you're out of a GPU for almost 2 weeks)
Worst Case Scenario (You sell your 2070 in a few days, have to wait 2 weeks to purchase a 3xxx - miss out, have to wait until octoboer for a 3070 (maybe you miss out again) all while being without a current GPU.)
If I'm you - I Try to buy a 3080 while i still have my current GPU - Sell the old one once you're a few days away from delivery date. Safest bet if you've got the funds laying around - Maybe try and sell your old card to a friend for the cash upfront or, swap for their old card + cash so you atleast have something in your system.