This is always a good question. I don't think there is a hard and fast answer, but going with a non-auction format, i would probably start at around 30% off original price. If someone were to buy used vs new, the savings have to be significant to warrant a used purchase to overcome the benefits of new. For example, with Axioms, shipping is free. Buying used, the buyer pays for shipping. This is one example of a 'used' cost addition buyers incur and hence think about before making an offer.
I have a friend who doesn't buy anything used unless it is 35% or LESS than its original price. Mind you, he tends to collect more vintage items and have been known to go as high as 50% of original cost.
The other consideration is brand name (and age of the unit). In this finicky audio world, many hold certain brand names in such high regard that they hold their value quite well. Bryston is an example of such a company. Krell another.
I've notice from sales of Axioms on the used sites that they don't do too badly. However, as stated previously, any unit with only 20% off or less barely matches the price of a Factory Outlet sale from Axiom.
On the plus side, if you put up an ad on Audiogon for the speakers at 30% off, do this twice, and it doesn't sell, you can always put out another ad with a lower price. It is not like ebay where you pay for every ad entry through the nose. If you peruse some of the 'high end' name speakers (e.g. Tannoy), you will find some supposedly
amazing deals. Speakers that originally sold for 24k could be going for as little as 11 thousand!!
If you want to try out a Cdn ad site, i suggest
Canuck Audio Mart. You may find selling a Cdn speaker a bit easier there as more ppl might be familiar with the Axiom name.
If/when you get around to moving on to another kind of speaker, you should so everything possible to do an in-home comparison between your Axioms and the new contenders. A good A/B comparison will remove practically all doubts from your mind as to which speaker you like better. 
I personally like the Axiom 60's-80's sound over the Klipsch RF35 through RF7 speakers. Horns just aren't as accurate to simulate how the artist would sound. Don't get me wrong, they sound good, and are good for head banger music 
