From my personal experience that sort of stuff doesn't happen anymore due to aggregators. Prices do change based on the moment you searched, but I've constantly been getting the same price across ITA Matrix / Google Flights / Airline Official Website / Priceline
No Incognito or VPN needed
What I do still see changing based on different metrics like IP are car rental prices and hotel prices
All US car rental companies give much cheaper prices to foreigners. I don't know the reasoning but its absurd, A rental car in Manhattan for Easter weekend will cost you $300/day on kayak.com and $70/day at the same exact branch on kayak.com.hk (this also works on the companies official websites when changing "country of residence")
No Incognito or VPN needed
What I do still see changing based on different metrics like IP are car rental prices and hotel prices
All US car rental companies give much cheaper prices to foreigners. I don't know the reasoning but its absurd, A rental car in Manhattan for Easter weekend will cost you $300/day on kayak.com and $70/day at the same exact branch on kayak.com.hk (this also works on the companies official websites when changing "country of residence")