I won't advice on price because you are in the States and that's a different world when it comes to buying a second hand saxophone, you are blessed with a larger market and a lower prices than us outside the U.S.
Mark VI's are not at all rare both in Europe or the U.S., their status of collector's items is based on a legendary sound (true for some and less true for most) and the fact that many of the great players use-d them but not really based on rarity if that was the case you should pay way more for other much rarer and equally good saxophones like SML for example.
200.000 assorted Mark VI were made, and that is hardly scarce although not common.
I can find them at any time advertised locally in Holland, mostly by shops or professional sellers but not unusually advertised by private sellers. Lately I have bought two altos , the price was reasonable.
These days there are a lot of folks out there in need of a cash injection and they are prepared to sell their saxophone for a much more reasonable price than you would have paid a few years ago. It is just a matter of having the cash ready and react quickly when one of these horns comes by.
If you find one advertised locally phone, agree to a price beforehand, go there as quick as you can, have the money ready (if you are scared that someone might rob you make an appointment somewhere safe........it never happened to me but the thought has crossed my mind that someone would wait for me with a gun instead of a saxophone knowing I have a large sum of cash on me! ). Ask for a receipt if remotely possible, say you need one for insurance purposes.
If you buy on line you have to factor in the possibility of repairs and the fact that you never know if everything is to your satisfaction. I wouldn't worry too much about intonation issues which are mostly cause by the player himself (unless someone has tampered with the key height ).
Mark VI baritones have been made much longer than other VI's because there was never a Mark VII baritone, so, maybe you can find one that hasn't been mistreated although being baritones mostly played in marching bands in the U.S., the chance that they have survived unscaled is minimal. So, for a baritone, expect damage, if you find one undamaged you are very lucky!
If you don't mind a later serial number definitely get one late, for some reasons people think that saxophones are the only industrial product (because they are industrial products, even when they are so called " hand made") which was born perfect and gradually became , purposely, worse as production numbers rose . This is , of course, against any logic but so is much of the legend around these horns anyway.