I often use the RM collection service for bulky and heavy items (the one where they bring the label too for an extra 30p) and they've never let me down. I've never tried this via eBay though, so perhaps it's different, we do see a few threads from sellers whereby they've booked a collection and nobody has turned up.
Not something I'd risk for an eBay order, tbh, I'd take the item to the nearest PO and send it out from there, though I do appreciate that some sellers may be unable to do this, for one reason or another.