It's luck. A Viking custom, carried on in some parts, usually in the north. I'm so used to it in our local marts that when I started buying and selling Shetlands, with folk from all over the country, I was shocked at the lack of it.
Sometimes the seller gives the luck to the auctioneer to give out, as they haven't stayed to see their stock sold, or the auctioneer stands a better chance of seeing who bought it.
Occasionally you get luck 'in kind' rather than in money. When I bought a field from my neighbour, I got several tonnes of limestone chippings :-)