I run across massive stamp collections at flea markets but only buy when they are super-cheap and have sports subjects. Last one I picked up was a book with hundreds of foreign stamps for a buck for the whole book.
The only other stamp items I buy are the various commemorative ones of boxers, again, if they are really cheap. I have a bunch of Ali, Frazier, etc. Here is a 1964 Slania Clay
This is a knock-off 'Slania' Clay which, ironically, is much harder to find than the genuine article:
Probably the most I spent was under $10 each for a pair of Olympic sheets of Zou Shiming, the first Chinese boxer to turn pro. Terrific amateur who turned pro relatively late but had a nice run until an eye injury took him out.
Those signed covers are really nice. Definitely something to keep an eye out for.