I don't agree, but I'm happy to see someone spout this nonsense. The thrift stores are becoming over-run with the middle classes clamoring, apparently, for new goods from Target at very slight discounts. It's like letting the PTA into the carny world (and while I may be a member of both I don't necessarily advocate mixing).
I'm particularly amused by the foot-molding argument. I never cease to be touched by the charmingly naive belief that we are all so very different and that one size 9.5 could never be compatible with another one.
The key to buying used shoes (and any used goods, for that matter) is to simply not buy things that are used up. I too would pass on heavily-worn shoes from a heavy person. But bring on the high-quality shoes from a sprightly guy's fifty pair rotation!