Rossetti Piano

 I copied this from an older prior post.  Some of it doesn't apply in
your case but its still good info and explains why it is so difficult
to give a value on an accordion that you can't see - both inside and
outside.  If you really want to find the true value, you will have to take
it to an actual accordion repair person - a professional who knows the
time, cost and extent of restoration required.  Also, you should expect to pay an evaluation/estimate fee for this service.
It takes time to disassemble and evaluate the instrument and no one
(or hardly anyone) ever works for free anymore.  Also the person doing
the evaluation takes a chance on damaging something when he opens the
box - the gaskets come apart, glue comes apart, pieces fall out and
the customer doesn't believe that the damage was already there or
incidental to opening it up.
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Some things that would need to be known in order to value it:
The actual manufacturer
Kind of accordion - Piano, Chromatic, 1 row, 2 row, 3 Row, Cajun,
Bayan,  Bass, Anglo, Concertina, etc.
Number of Keys Bass and Treble
Size of treble keyboard 17 1/2 inch, 19 inch etc.
Tuning - musette, dry, in tune or not. Accordions may sound up to 8 reeds for each note and all must be in tune
State of the leather valves - flexible and soft, hard, curled, etc.
Wax condition - Reeds are usually held in place by a mixture of Bees
Wax and Rosen - accordions stored in either cold or hot temperatures
or in very dry areas usually have wax that has crystallized, melted,
or dried up and will need to be replaced
Condition of surface, warping, cracking, rot, etc.
Type of reeds (hand made, hand finished, factory) and condition (no
rust or corrosion)
Condition of the felts under the keys
Number of reed banks - treble and bass
Shifts/registers on each side
Bellows condition - is it lined, how many folds, condition of glue and
corner leathers, gaskets
Any musty odors or bugs inside - Don't laugh - mealy bugs and moth
larva cause havoc in an accordion
Condition of the bass machine - rust etc.
Condition of straps
Case usually really doesn't matter unless its really unusual
Appearance - does it look good.
Playability - ease of use, balance, key travel and pressure required,
etc.
Accordions generally have little value as antiques unless there is
something special about them - like it was used by a very famous
accordionist and it is documented.  Or if it is the first of its kind
or is really old or is signed by a famous maker
Part of the reason for this is the cost of restoration.  If an
accordion has set for 15 or more years without being played, it almost
always needs cleaning and tuning and some restoration.  For a full
size accordion this costs upwards of $400.  Perhaps even more than
$2,000 for a serious and complete restoration.
So what happens is that an accordion that cost $300 new in 1940 may
require $700 in repairs in order to get it working properly and then
will have a resale value of only $400.   Just not practical...  Lots of
these 40's and 50's vintage boxes go on sale on ebay and end up
selling for $40 - $50.  Basically they end up as parts or something to
play on outside gigs if it looks like rain....A few old accordions are very well made
with excellent reeds and potentially could be worth more that a new one
if completely restored but only your repair person knows for sure. I have seen
a few restorations in the neighborhood of $1500 that resulted in a like new
box that was better than 99% of the new accordions out there that cost three
or four times as much. The problem is to find a technician that can do that
quality of work.
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Hope the above info helps. Also it explains why such a simple question
like "how much is this accordion worth?" can't be answered easily.  It
is not that no one wants to help, it is that the answer is very
complicated and an answer like "Anywhere from $50 to $5000" doesn't
satisfy the question.  This question comes up very often on this and
other accordion forums and unfortunately the answer is very rarely
what the person asking the question wants to hear.  Also unfortunate
is that there just are not that many knowledgeable places, at least in
the US, where you can take an accordion for evaluation.  The average
music store/instrument repair place doesn't have a clue.  Sometimes if
you’re lucky, the local accordion club will have a member or two that
can help.
John C.