If you are selling, it literally takes about 5 minutes to google Interparcel (or other provider), input the details of the items (size, weight) you are selling and the time/date you have availability for collection, get a quote and either proceed or shop elsewhere if its unacceptable.
If the buyer has to arrange the courier, they then have to find out the size and weight of the package from the seller, get all the address details from the seller, and find out when the seller can make it available for collection - all of which requires the seller having to provide exactly the same information as if they arranged the collection themselves but requiring an intermediate series of messages. The seller still has to do by far the most time consuming thing which is to pack securely and label it.
It just seems like unnecessary additional time and effort to insist on the buyer arranging the courier, as far as I am concerned. Literally the only benefit I can see is that the seller doesn't have to pay the delivery cost as part of the overall financial transaction, but this is usually a small proportion of the total value.
I just don't get it