I have seen a lot of "used cams" installed in my days. Very true, it is best to install new cam and lifters. Money dictates a lot of things when you are in need. I have never seen new lifters wipe out a used cam. However, if the cam has substantial wear already, nothing is going to keep it from wearing, whether new lifters or not.
If the cam is in good condition and the lifters are marked from their correct position and I needed a cam I would use them. The question is, if the cam is in good shape, why did the guy remove it in the first place? If he was putting in an upgrade cam it is probably not any better than you have now, unless yours is worn out.
By the time you consider that cost of gaskets, new antifreeze, timing gear (if I am going to have the engine down that far I certainly am not going to use the old timing gear), and time and labor doing the work????? There is a lot of consideration on doing this.
Of course, in my younger days, I have reused all of the parts before except gaskets. Like I said, $$$$$$$

Ed