Well…
Shamisen can easily last decades if well maintained, and the skins can last ten years even with use if cared for properly.
As an example, here’s a refurbished 141 year old shamisen being played by an extremely skilled lady out in Nagoya.
I wouldn’t really be worried about ageing, especially if you don’t mind occasional cosmetic troubles. All of my shamisen were purchased used.
That said, it is possible that you might buy this only for it to break in transit or for the skin to pop shortly after arrival. Repairing damage due to transit or replacing the skin could very well bring your costs up and negate much of the deal you’ve found.
If you live in Japan and don’t need to worry about international shipping, I’d say “yes, without a doubt”. My experiences are 95% positive, and that tiny 5% was a great teacher.
If you live elsewhere, it’s harder to advise to wait or jump on it. Maybe some other folks that bought via etsy or ebay can comment on their experiences.