I'd put the thing in a really strong box (and I mean really strong) and pack it extremely well with LOADS of polystyrene, newspaper, bubble wrap and those air cushion thingies and whatever else it took to survive being dropped from a first floor window (you can expect this with most couriers).
I'd then get a quote from parcel2go or senditnow as they cover stuff up to 31 kilos at least, often very cheaply.
Check it isn't on the 'restricted items' or 'no compensation' list when you book, or they'll drop it and it'll break and you'll lose everything.
OR get the buyer to arrange their OWN courier but you will need to know their tracking number before you let them take it, so you can prove it was delivered ifthere's a paypal dispute. Online tracking/proof of delivery is what you need.
There is always a risk in selling heavy stuff as it might get damaged...I just got a car seat collected, I arranged it myself but the seller packed it badly and the box broke on the way. LUCKILY it is just about fine, however, had it been busted I'd have lost my money I think. (hard to claim for damage from parcelforce unless something is packed to within an inch of its life - and this wasn't!)
Soooo just be careful. Offering collection only is a good way to start, you'll get a lower price but at least you'll know the thing is safe.