Last year I finally decided to change my 70's Mark VI for a 60's Mark VI. I was hesitant to do this because I had never tried a Selmer that was better than mine. It just wasn't doing what I wanted it to do. It was a little weak in the high and low register and I couldn't blow as much air through it as I wanted to. Ebay was much too scary for me and I had tried all the horns in the Northwest that I could lay my hands on. I even drove from Florida to LA on a sax quest with no luck at all. Finally I called Mark at Saxquest in St.Louis, Mo. He played a few over the phone for me and then sent me a 1955 Mark VI in almost perfect condition. The horn looked incredible but played way out of tune. I decided to try once more (each try costs about 100 dollars for shipping and insurance). The next horn that he sent was the horn of my dreams. It had a huge rich, smokey, clear, dark sound and it played in tune. Mark changed my life in a big way. He's trustworthy and easy to deal with. He also traded a few old Kings for a great Mark VI for a student of mine. I would recommend Saxquest to anyone who is in the market for a vintage horn. They also have about the best prices on reeds you can find on the web. Here is their links page with plenty of good sax related web page links.
Some other reputable sax dealers with good inventory are:
Tenor Madness - Randy is a master repairman and you'll get a horn that has a great set-up.
It's always hard to even tell if you like a horn if it isn't set up well. He has some great Otto Links that have been refaced to play like old slant signatures for $200.
Junk Dude- lot's of great vintage horns at reasonable prices.
World Wide Sax