In the past 3 years, I've bought 8 horns off Ebay, 6 of them saxes (also a flute and a Clarinet).
The first sax I bought was a Conn 6M, and I was surprised that the horn played very well when I received it, but it needed some work. Still $300 for a 6m?!? What a great deal!!!
Sax # 2 was a Conn New Wonder Alto from 1922. This one, I paid $200 for. What I got wasn't worth $50! Many of the keys were rusted and frozen. At least there were no dents and no other damage. I figured the horn would need a cleaning and a repad, but it ended up needing a lot more work than was indicated on the ebay ad. I let the instrument soak in Kerosene in my garage for a couple weeks before attempting to remove any of the screws or rods. Luckily, I was able to free up the action and most of the screws and rods came out without any trouble. Only one of them really gave me fits, and I had to heat it up in order to get it out, but it finally came. I've completely restored the horn now. All together I have about $700 invested in the horn. Probably more than I could get out of it.
Sax # 3 was another old Conn; a New Wonder C Melody. This one was in really nice shape, and I did a "buy now" for $300. I was a little surprised when I got it home that it actually played quite well. Some of the pads had been replaced, but some of the larger pads WERE ORIGINAL!!! 1922!!! This horn polished up to look like brand new, and I think that most likely it spent the majority of it's 83 years tucked away in the back of a closet. I've replaced a few more pads, did some adjusting and this sax plays very well. In fact, when I'm just playing around here at home, this is the one I pick up and play most often. I've just started playing some Christmas music on the weekends with a quartet and I've been playing the old C Melody.
Sax number 4 was most definately a rare find. The add said "Old Conn Saxophone, Alto? No reserve. It wasn't an alto. It was an F Mezzo Soprano. I recognised the sax in the photo, and the seller had also listed a picture showing the serial number. This horn needed some attention due to it's age, but was in very good shape overall. I bid $500, thinking surely another buyer would know what this was and outbid me at the last minute in the auction, but I won the horn for $470. I've had it repadded and cleaned. It's more of a collectors piece that I don't play often.
Sax 5, a 1955 King Zephyr Baritone, I bought for $2500 in near-perfect condition. Sax 6, a Conn Chu Berry tenor, needed a good cleaning and an overhaul, but $490 for a Chu with no dings or dents didn't seem too bad at all. The flute and clarinet were like new.
As for my experience with ebay, I can only offer you this advice... READ THE ADD THROUGHLY AND ASK LOTS OF QUESTIONS!!! BEFORE YOU CONFIRM YOUR BUD, MAKE SURE YOU READ ALL THE FINE PRINT!!! Also, make sure you know what shipping is going to run before you buy, and be aware that some sellers charge extra for shipping and handling. Some quite a bit extra!!! I had a seller want to charge me $24.95 to ship an ALTO SAX MOUTHPIECE!!! Yes, just a mouthpiece! And the Bas@#$d gave me a negative when I refused to pay more than $5.00 for shipping!!!
I check ebay every couple days just to see what's out there and what stuff is going for. As a collector, watching the auctions to see what the horns are selling for gives me a more realistic idea just how much some horns are worth. Many people ask me how much a certain sax is worth, and the value really depends on the condition of the horn and rather there is a buyer who wants it. Often a particular model will list for high $$$, but finding a collector willing to pay is another story. Then, on the flipside, I've seen some less valuable horns in really nice shape bring MORE money than I ever would have guessed, because a couple buyers really want it.
Finally, I would say that ANY old horn you buy off Ebay or from an individual, plan on putting some money into it to have it adjusted at the very least, and most likely pads and a good cleaning.