I have been browsing the accordions on here for sometime looking for interesting items and have noticed very similar deficiencies in people’s listings. I am no expert but I am a player and I tinker with accordions at the hobby level. I know that many of the sellers listing accordions on here have no clue what they have or anything about accordions (few people do) but I thought I would write this and help you out. I'm sure this will help you avoid the repetitive questions that most people need to know before buying an accordion. So here is a list of items/info you should include in your listing (and maybe in your title for easy searching).
1) The number of keys. Count all of them on the key board (white and black) and count all the basses (the little round buttons on the back/left hand side) a full size accordion has 41 right hand "keys" and 120 left hand "basses". 2) The size of the keyboard. Measure from the distance from the outside of the outermost white keys. A full size accordion is 19". 3) the number of shifts, these are the buttons over the keyboard (or sometimes palm shifts at the end of the keys) as well as the buttons over the bass buttons. If there are symbols take a clear picture of them or if there are names ie flute banjo list them. This is important because this will help the buyer know how many and what octave ranges the reed blocks are. 4) The condition of the bellows. Cracks, leaks are something that accordion buyers will either accept or not. Some will have no problem fixing these types of things others may not. 5) Odors. Old accordions are well old and some will have a musty odor. 6) Noises. Put on the accordion and pump the bellows. Are there any noises when no keys or basses depressed. If so tell the buyers. This will avoid negative feed back. And also test all of the buttons and keys in push and pull. I know this is a pain but it may get you a better price if you can say honestly and with confidence that the thing works. 7) Tuning. If your not musically inclined this is not easy but try playing the thing with all the different shift combos. If you here a tremolo or vibrating tone you may have what is called a Musette or wet tuned if not it is dry tuned. Both tunings are wanted by different players. 8) of course name, make, model, weight ect are also important but there are so many accordions out there and very little info in those types of things just listing this is not enough to get any stats that way. In other words if you can't find it neither can you buyers and it not like a car that somebody is usually looking for a certain make or model. They are looking for a certain sound or size of ...
 If you feel comfortable take the little pins (nails) that hold the bellows out and take pictures of the reed blocks on both sides. Look for rust and missing reeds. If these are in good shape and you have the right combo of reeds this will mean bigger $$$$ but BE CAREFUL NOT TO TOUCH THE REEDS or mess with them in anyway unless you really know what you are doing (which if you are reading this you most likely don't) Just be careful if you decide to take this step.
I probably have forgotten some items but this guide is a good start, and will help get the right accordions to the right buyers.  Happy selling!