LOL the mother of the book store owner have "compulsive shopping disorder" :)

The "Sell bitcoin for USD" is a conversion method on the site or in their app. Paxful has three wallets: BTC, USDT (ERC-20 and Tron), and ETH.

The video didn't demonstrate how after the bitcoin was converted to USD, what else happened so that she ended up with cash in hand to use for spending at the bookstore.

Oh, now that I think about this, maybe they are promoting the use of converting to USDT/Tether simply as a way to protect against exchange rate volatility. Like, if she had cash already, and just holds the USDT received. Or maybe she did another local, in-person (face-to-face) trade selling the USDT for cash? Selling Bitcoin instead of Tether likely gets you a better, high price though. Who knows.

Now if the bookstore had accepted bitcoin the two conversions, first to fiat, and then back to bitcoin, could have been omitted.

Paxful even has Lightning network ⚡ support (for deposits and withdrawals) so the "bitcoin is slow, fees are high" argument would not apply.