If a buyer asks to cancel, Ebay offer a short list of reasons - I would guess your buyer chose one randomly.
That is an expensive item for a brand new seller to list.
I can see you are protecting yourself by using RM Special Delivery but there are a lot of bad people out there So, if you do get another sale, check the buyer's feedback - feedback received and they have left for others.
If they have their feedback set to 'private', personally I would walk away.
If they leave negatives claiming items were not as described on a regular basis, don't deal with them.
Check it is an established buyer with a recent record of similar purchases.
Above all, be careful and 'trust your gut' - horrible expression but apt when selling on Ebay.
"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)