More postage questions

My car failed it's MOT, it has a bull bar on it, I thought I'd sell it.

I have people queueing up to buy it, if I can post it. Mine is £150 less than anyone else, who offer free postage, so why can't I post. Ask the couriers I say, it actually states that on the listing, because of it's size and weight, it will cost £116.50. When you tell people that, they say they could drive down and collect it cheaper, that IS why it is collection only or arrange your own courier.