I would use the term Hand or Keeper
Agent could work too.
Nobility generally had servants to buy things that could damage their social standing or reputation. Typically, personal vendors, hand-servants, housekeepers, servant managers etc... Essentially, intermediaries or intermediaries of intermediaries
whether or not the items were too base( How boorish) Put you in suspicion(what are you a criminal?) Or just plain weird as hell (what do you mean he wants 36 watermelons cut at a 34 degree angle and heated to exactly 89.5 degrees)
These vendors would be given a purse and instructed to buy whatever the noble in question wanted. Provided they didn’t draw attention to themselves, attempt to rip off the noble, steal and or ripoff said noble or noble house they were pretty much intentionally obscured and you’d never know the person next to you might be a fellow-commoner or a vendor/servant of a noble.
Depending on how snobbish the rich are. (Like servants are not meant to be seen) To (Why is this commoner talking to me?) to even (Holy shit! poor people are real)
There might be several layers of intermediaries each with their own functions regarding to Law, Trading, Storage and Delivery, Personal Items, Arts and other luxuries. Etc...
There could be an entire complex social industry powered by the nobility for the express purpose to remove the nobility altogether from the common day to day. Like celebrities on steroids.
So far removed the common folk that they simply can’t conceive non-nobility life.