Measure accurately but get it as small as possible 14kg is normally about £12 but because likely to be outsize ,maybe upto £30 get some quotes online
make sure you state that highlands and islands of scotland / northern island channel isles isle of man etc are extra; ask them to send their postcode at least 24 hours before auction ends for you to check
select heavy bulky item instead of domestic postage flat rate
do not let buyer book own courier as you will have no proof of delivery
have sent antique cast iron sink weighed 50kg and had garden tractor done with courier its fine,
get 2-3 giant crisp boxes to make 1 really big box, or similar from supermarket about 10 old newspapers for packing edges etc and a reel of brown parcel tape
good luck as for anyone more than 100 miles away it will be cheaper to post and will get yuo better price as larger market but of course they can not try before buy so more possible to get complaints and having to refund
you could security mark it before sale too