Here are my thoughts:
1. Don't get a used 760. Get a used 7600. It contains a few improvements, including a longer magazine release latch, which is easier to operate. Also, the magazines that are made today are for the 7600, and many of them will not fit the older 760s.
2. I have read alot about the Triple K 10 rounds mags. Apparently, they are pretty poorly made, and only about 1 in 3 will work properly and reliably. I noticed that in my last Cabellas catalog, Cabellas stopped carrying them. (They probably had 2 out of 3 sent back for so long that they got tired of it.) So, if you buy these things, prepare for some frustration. Some folks who got ones that didn't work were able to fiddle with them, by bending the lips or filing it here and there, until they did work. The ones that do work without alteration seem to be pretty durable.
Regards, Mannyrock