I disagree that a vintage bag will retain its value..bear in mind that buying preloved you are paying at least 50% more than you would get selling the bag through a pre-loved consignment store..
True in many instances... though I've bought H from auction and sold for higher (over and above auction price + premium + VAT on the premium) to one of the preloved / consignment stores. My 'day job' is doing the same with Tiffany and other designer jewellery regularly as well, both to consignment and on eBay. Bargains do still exist at auction (although not as many as when I was working at an auction house back in the early 2000s - I miss those days!!).
My latest purchase is a 2006 Casaque Rouge B35 in Togo, which I paid £5,737 for including premium. I've been offered £6,500-7,000 at various consignment stores. That includes a pen stain on the lining, which is going to cost me £300 to have removed / restored, then I estimate a higher consignment value.
Maybe I've just been lucky.
Also do bear in mind that any item purchased at auction you have to pay 25%+vat on the hammer price..So a £5000 bag becomes £6250 + the vat on the £1250.
Bonhams is 27.5% + VAT at the moment, so can be even higher. Also need to watch as if the bag is sold at auction by a business, you need to pay VAT on the hammer price as well as on the premium (there's usually a specific mark in the catalogue to indicate this).
If you want something that's not officially "vintage" - i.e. less than 8 years old or so - you're probably more likely to get it at a consignment store in brand new condition. Generally, the only brand new, mint bags that tip up at auction are slightly older.
(Eg: my latest baby is a Lindy 34, brand new, box, dust bag etc, but from 2013 - suspect if she'd been 5 years younger, she'd have done better on consignment than at auction).
Something random I thought of, given you may return to the US... Bear in mind many sold at auction don't have CITES certification, if in an exotic leather (definitely for croc, not sure for other exotics?) - they will say this in the Condition Report or description. This might be a problem, if you want to travel back home to the US with it.
I agree that it may be cheaper if you find the bag you want at a consignment store, and you trust them enough to either buy from them, or trust they'll refund if you get the bag authenticated and told it's a fake.