Crazy, isn't it? Anyway, I searched around the net for information about the instrument; I found surprisingly little, but the website of Christian Mensing got me started. It's a must read. Also, the youtube channel HAranguiz contains many videos of bandoneon players, including some quite inspiring footages of Astor Piazzolla.
So I started to play around with the idea of buying a bandoneon; unfortunately a typical 142 tone bisonoric instrument is fairly expensive, and it's also very difficult to determine the quality without already having the expertise, or having a physical access to the instrument for that matter. There are some good resources (see here and here) for learning about how to check the quality, but I faced two big problems.
First, without having any basis of comparison I would have a hard time deciding whether I actually understand these instructions. It's like when you have zero experience with cooking and you try to follow a recipe for the first time; the result can easily be a disaster, but you won't know until someone tastes it. Without having a prior feel for how a good instrument should work - i.e. how much pressure you need to apply to get a certain sound out - the only thing I could really check is the consistency of the behavior. But even if I had a feel for how an instrument in good condition behaves it would be still difficult to tell how much deviation from this normal behavior is acceptable. You can take the presence of such deviations for granted as most of the old instruments on the market were produced in the 1930's and practically none of them is in perfect shape.
Second, these quality checks assume that you can actually hold the bandoneon in your hands before you commit to buy. Unless you stay in Buenos Aires (or maybe in one or two other big cities around the world) you can't simply walk into a musical store to buy a bandoneon, because they don't carry them. Local owners - if there are any, none lives in my current area - are unlikely to sell. So your main option is to buy online. You can always find bandoneons on eBay for sale - the starting bid price for an AA with aluminium reed plate is around $1700 -, and sellers typically post many images and even videos to assure perspective buyers. Another online source is the bandoneon mailing list on Yahoo - you certainly want to sign up and browse through the archive, it's a gold mine - although the seemingly more trustworthy sellers who I contacted through the list tend to pitch their prices much higher (one asked 3600EUR for a recently tuned AA in perfect shape). The problem with online purchases is that there are many potential mechanical troubles which are difficult to determine on the basis of videos, like how well sealed the main valve is. I was advised against buying online; during the past couple of months I certainly saw a few seemingly really good deals, but you know the joke about the two economist who see a $100 bill on the sideway: one wants to pick the bill up, but the other holds him back saying that if it were real someone else would have already taken it. And so I kept looking around, counting my pennies, and salivating.