Wilson Wilson
Bluelighter
I prefer this site, has better security measures and fewer sellers require ID and selfies and shit:
localcryptos.com
As for these fancy "challenger banks" I would never buy BTC from them if I actually wanted to keep the BTC, it's pointless for the reasons given in the Reddit post that was linked above. Basically they make it impossible to transfer it to another wallet.
There is one called Cashapp which is more like a Paypal competitor and they technically do let you transfer bitcoin out but many people have reported their accounts being banned as a result of doing so and without any reason given.
But if you're just wanting to buy high and sell low, you should be alright, just don't use these apps if you want to ever spend the actual crypto.

LocalCryptos

As for these fancy "challenger banks" I would never buy BTC from them if I actually wanted to keep the BTC, it's pointless for the reasons given in the Reddit post that was linked above. Basically they make it impossible to transfer it to another wallet.
There is one called Cashapp which is more like a Paypal competitor and they technically do let you transfer bitcoin out but many people have reported their accounts being banned as a result of doing so and without any reason given.
But if you're just wanting to buy high and sell low, you should be alright, just don't use these apps if you want to ever spend the actual crypto.