Selling my Rotax 912ULS - Price?

" I see there is a wide range of what these things go for. "

Your asking (probably receiving is more important😁) price will depend as always on:

 # Maintenance record - should be full & complete (all actions routine, damage, remedial, listed). If there is no record or it is too minimal, this will have a negative impact on price."Carbs/Gearbox recently overhauled" great! but will need details of work done & name of maintenance facility."Ran on auto fuel 99% of its life" - Good! speaks volumes. Oil consumption figure needed.

 # Recent leak down & pressure test - NUMBERS within Rotax acceptable range will maintain price. Numbers below will depress pricet."Great compressions" means nothing.  

 # Number of hours flown against years in service. Low hrs/annum is attractive to some but not all  purchasers. "1100 TTSN - 2013 Model" good start but needs more detail 

 # Is it in/out of the engine crankcase fretting series. You will need to provide the all important serial number.

 # Does it have "soft start"

 # Are you selling a complete engine, including carbrettors, airbox/ air filter(s), exhaust system or something more/less. Detail?

  # External appearance.

While it would be a bonus to hear/see it running the above points will be more important to the discerning buyer

As price will vary from country to country, best that you perouse the aircraft classified/advertising services in your are, to get a handle on what people are asking.

Since it is a little more than halfway through the specified TBO, I would think that if everything else is fine, that 1/2 the price of a new engine would be the top of your asking price range.

"Since it is a little more than halfway through the specified TBO, I would think that if everything else is fine, that 1/2 the price of a new engine would be the top of your asking price range."

I stand to be corrected; The reason for this engine being offered for sale is - it has or is about to, run out of calendar TBO. While this engine could easily continue to be used (on condition) it can not be used in an aircraft that is being offered for hire or reward.

So your statement  "..... little more than halfway through the specified TBO" is probably not correct, therefore your suggested pricing calculation may also be wrong.