The market for used pianos is one where supply can exceed demand. The serial number of your piano indicates that it was built between 1912 and 1913 (see: http://www.bluebookofpianos.com/serial1.htm#ARMSTRONG) and in great condition, with only minor adjustments needed, comparable antique upright pianos sell at the $600 to $800 level. If it needs some work to get to current playable condition, the value would be much less, likely at the $150 to $300 level. An issue is that pianos from this era have some inherent technical issues which can't be fixed to make them sound like today's pianos. As such, most teachers don't want students learning on them, which obviously limits the market.
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