I didn't read all posts but the OP and first couple of responses mentioned the term "retail price" which is open to interpretation. The price that is charged by retailers and can fluctuate wildly. An astute buyer is patient and waits for a rock-bottom sale which already can be 50% off the "regular" long term price. On the other hand, if you are not in the US and have an instrument shipped from let's say the Ukulele Site, even a rock bottom price can easily inflate substantially with shipping, customs, and taxes.Someone who wants to buy a used uke may not be aware of the additional costs but only of the rock bottom price that may be posted elsewhere. So rather than referring to "retail prices" and trying to calculate % deductions, it may be a better exercise to set the price on something you find acceptable in you mind and then add a 20% (or whatever is customary in your area) haggling factor to the asking price. This way you can make sure that you get at least the price you really want and the haggler feels successful in lowering the asking price, as this is more about psychology than money.