One who buys. Named by the transaction, which is why this house prefers vessel or participant where the relation is more than that.
Old French costume (habit, custom), from Latin consuetudo (usage, habit); originally one who habitually deals with a particular business, narrowed over time to simply one who buys.
The word once named a habit, someone who dealt with a business regularly, before it narrowed down to naming only the single act of buying. This house reaches past that narrowing wherever the relation is closer to the older, habitual sense than to the transaction alone.