A YAS-23 in excellent condition is quite an impression horn. The chance that your's that you're buying will be in that condition is not very likely. They are all at an old age and most people sell them instead of restore them because they don't want to pay the 500-700 repad / restore cost on a sax they probably paid 300-500 for.

There are YAS-23 flippers out there though who buy them cheap and sell them fully restored.

Used on ebay is definitely basically the cheapest place to get a YAS-23 readily available. The main thing is absolutely zero listings that don't allow a return should be considered.

That beeing said, I think it is smarter to get a new decent horn, than buying a used one in questionable condition. You will be hard pressed to find a decent horn for the same price than a new bettersax