Owning your own home

tradesman is sat around whilst arranging and waiting for the courier

Tradesman orders parts from supplier, supplier organises courier, tradesman gets on with other things, profit. Except in the case of a small repair job, it's unusual that the shortage of one part would leave the tradesman with nothing to do on site, but even then the likely proximity of the courier to the parts supplier means the part is usually on site sooner by using a courier, unless the warehouse which has the stock is very close to the job site.