Every sale transaction records which buyer took the lot. The ring clerk enters the winning buyer (usually by buyer number) the moment the hammer falls, and that buyer is who gets invoiced.
The buyer must already exist in the buyer registry before the sale is recorded — you assign the winning bid to a known buyer, not a free-typed name. This is what links the lot to an invoice and a loadout.
Technical section
Data model, API, and internals — visible to developers.