If you want to sell something and aren't sure what to charge, the best answer is probably "whatever someone is willing to pay."
If the item in question is still readily available brand new in the marketplace, then I wouldn't try to sell it used for the same price as if it were brand new from a store, since anyone who wanted to buy it could easily spend the same amount of money as you're asking (except maybe for sales tax) by going to a store and buying a new one. In that case you might look to see how much stores are charging for the same item when it's an "open box" or "returned" item, although the condition of the item is also a factor. Is everything working perfectly or not? Are there scratches or other blemishes? Are there any extra accessories thrown in?
If the item in question is no longer available brand new but is still readily available used, then you can look at how much people are asking for it. As you've discovered, the asking price might vary dramatically, and in some cases the asking price might be completely unreasonable-- e.g., I've seen keyboards that originally sold for $100 being offered for sale used for $200 or more, even though follow-up models having more voices, styles, and functions are readily available for $100 or less. So an important factor in helping you set your asking price-- in addition to the condition of your item-- would be whether or not the manufacturer still has follow-up models for sale in the marketplace, and whether the follow-up models are considered to be better or worse than the older model you're wwanting to sell.
But getting back to the idea of "whatever someone is willing to pay," you should ask yourself what is the lowest price you're willing to accept, as well as what is the best price you would ideally like to get. You might also try to put yourself in the shoes of the potential buyer-- i.e., what price would you be willing to pay for the item if it were used and in the condition that it is? Then set your asking price at your "best ideal price," or maybe a bit above that price, as long as it isn't way over the price you yourself would be willing to pay, with the understanding that some amount of haggling might take place as a potential buyer tries to talk you down to a lower price.
The thing is, you don't want your original asking price to be so high that it drives away potential buyers, but you also don't want it to be so low that you're just screwing yourself. I don't know how rare or coveted the Roland AX-Synth is, but it's difficult to know what some "crazy rich collector" might be willing to pay for something.
Another question is, how anxious are you to sell the item? Are you looking for a quick sale because you need the money as soon as possible, or are you content to set a higher price andwait for as long as it takes for someone to finally make you an offer?
Sorry if I haven't given you any solid answers, but hopefully I've raised at least one consideration that you hadn't already thought of yourself.
