First it is not a collapsible stock, it is a folding stock.
Second I have seen the stock sold separately, but I'm not sure about the legality of retrofitting an M3 to the folding stock version, even an original preban, 7 mag, pistol griped version. I'd think now that the AWB is over it would be okay to reconfigure a preban w/o running into 944(r).
But hell everyone seems to be taking M4's and converting them to long mag, pistol grips, with functional collapsing stocks. I had one on order at Impact was one of the first, but cancelled my order when I realized it probably was not legal.
I bought a preban M3, and aside from the stock being way too long it is great. Super bad ass actually. I think I prefer it to an M4. ONE of my winter projects is to cut an inch or two off the back of the stock, fill in some fiberglass bondo and then put a new recoil pad on it. Wish me luck....
As stated all M3s have the recoil spring around the mag tube. (Don't ask me how to change it cuz I aint figured that out yet
)
I'd verify it was preban and try to figure out the configuration. $1,300 seems like a good price depending on the condition and whether you think you'd find the folding stock useful.
I really want to try a Mossberg 590A1 with ghost rings and a Knoxx Spec ops stock. That looks cool.