Collecting (modern) clarinets or saxophones is a losing game. In both of those worlds there is about one instrument each that have appreciated in value: for clarinets, that's a Haynes Thermocouple. For saxophones, that's a Selmer Mark VI. (Of course, I'm not talking about prototypes or horns owned by famous players.) There are more that retain their value ... but it's still only a few. It's also becoming a lot more difficult to find something on eBay that's listed incorrectly that you can "flip" and make a profit -- primarily because you can get info about the horn you're looking at on websites like this.
Now, if you're talking about buying a good playing horn for cheap on eBay, that's very possible. This is because people seem to think that a $1500 SML Gold Medal sax in perfect shape is inherently not as good as an $8000 Yanagisawa because the SML was made 40+ years ago. However, the problem is that you really need to have someone to guide you and tell you which old horns are junk and should be avoided. That's sometimes difficult, especially when the expert says something like, "Yup. That Selmanbuffet is good, but only between serial numbers 1 and 3.14. Or if it has silver keywork. Or if it was made by Sam Jones," etc.