Proceed with caution.
For me, buyers needing the item in a hurry can be a bit of a 'red flag'.
As advised, it is really easy to end the auction early and simply sell to the highest bidder.
Have a look at their feedback and feedback left for others to check there are no warning signs.
Don't exchange any personal details such as phone, WhatsApp, personal email.
Double check they have paid before you post by going to Payments
And, as advised by @plum993 use Royal Mail Next Day Special Delivery. It is properly tracked and, although it will cost you a bit more, it should ensure safe and swift delivery.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)