I greet everyone I know many of us wey dey here suppose don get better understanding about crypto-currency and even how to trade on binance but I still believe that this forum is growing and there are still many newbies coming in.
so to go straight to the point my friend who is a bit new to crypto-currency was telling me about here experience sell her Bitcoin a popular exchange she has already understood some of the basic on how to sell but this Time while selling the buyer asked for here number which she gave and then chatted her demanding that they trade outside the exchange, and even offered a higher deal than the exchange. this is a big red flag because you a vulnerable to getting scammed if you are trading with someone you don't know off the exchange no matter how juicy the offer is.
Since you mentioned that the seller was a newbie so I won't ask her some technical questions, but basically as you rightfully said this method of scamming I quite cheap but those without knowledge can still fall victims for this and for sure it will result into a big lesson for the victim if she fall eventually.
But to any knowledgeable trader that alone is a big red flagand shouldn't be taken off the exchange for whatever reason and if there is anything to ask or chat about the exchange chat box is there to make every communication.