I think it would depend a lot on what you are selling, especially if you are selling stuff that is likely to be sent as a gift, or where there is an expectation of nice presentation - such as jewelry, clothing, cosmetics, etc.
You never know what will set some buyer off, but usually that is going to be a 1 in 1000 case, or learning curve issues. When I switched from selling books to small industrial parts I had a few complaints at the start as learned what worked and what didn’t.
The only complaint I recall about used packaging was during the Anthrax Scare after 9/11, and that was informal (customer msg’d me).
As for as salvaged packaging. Fabric and Hobby stories are a great source. Especially the cardboard tubes that fabric comes on which is great for posters or long thin fragile items.
The worst problems where with the local Post Office where for a time they had a 2 windows clerks who would refuse to accept anything with extraneous marking - even to the point of rejecting new packaging if the company logo art was too big on the address side - but that was not a USPS thing, but was just them being difficult and trying to up-sell packaging material.