I trade quite a bit on eBay, and in the UK at least you can get more than 20% of the original retail for a relatively new horn in excellent condition, particularly if it's a desirable model. 20% of the new cost of a BW alto would be just £70, and you'd always get way more than that here. Usually 50% or maybe a bit more. For a well regarded budget instrument like a Walstein you should do OK, and you're helped by the fact that, now they've established their UK market presence their prices are going up. £200 is a pretty fair guess, I reckon. The key to selling well is to use eBay, have some good photos and a very well written advert, with good written English. Be honest and clear, and point out what the current new price of the sax is. If there is a level below which you don't want to sell, set a reserve.
If you do get £200 for it, it means the sax has only cost you £50 a year. A quid a week, which is pretty good value.
