For the most part it's still a gentle tradition, just a coin or two, maybe a fiver on a big sale.
Yes you do suspect that larger amounts pass across in the headline sales.
Sometimes a seller will leave 'luck' with the mart office, to be passed across when the buyer pays up. More usually the seller will seek out the buyer and pass over the luck in person.
I have several times been chased by buyers clearly wanting their luck money; on store lambs it was usual to give 20p - 50p per lamb, on fat lambs 10p-20p per lamb (usually selling in 10s at the least, some pens will be up to 50 lambs or even more.) It is an opportunity to show your gratitude to the buyer and stick in his/her mind, so that next time they are bidding on your livestock they may decide to go that extra bidding increment and you'll get your luck money back in spades!

I was once given a fistful of notes on a small pen of draft ewes; that seller has certainly stuck in my mind!

I have given and/or received 'delivery for luck', 'luck potatoes', 'luck hay', 'luck feed', ... it's a nice tradition and I support it - but I do know some farmers who feel pressured by it and won't do it.