how is postage providing more privacy? you need to put a return address which is ur address and ur name in writing and phone number also
cod just show up at a public place and be done with it

to OP
for high item like 6k, if its electronic their usually have an official shop to purchase from, if its car parts or wtv then idk la but for 2nd hand electronics 6k might as well go buy from official shop
but cod is the best method, go there, test all you need then exchange money then sau pei and dont bother the seller

i recently sold a relatively high priced item, did it via cod and the guy insist on cash when i want bank in, maybe he also dont want reveal name or something then we just deal, i didnt check if each note is real or not so i might be fucked lol