As in mostly everything in life, the answer to this is, "it depends".
With old films, there are too many variable factors to safely comment on. Case by case situations apply.
I recently found two army surplus metal ration cans with 40 rolls of B&W and a few C41 films I had stashed away and forgotten for - gosh, at least ten years, likely longer. Tucked away at the back of a cupboard. Summers can be scorchingly hot here in rural southern Australia and our winters, while mild to an expat Canadian, are also known for their seemingly unending cold spells.
Anyway, I took two test rolls, one B&W and one color neg, and ran them through my Contax G1s, making images under variable light conditions. Processed both. Surprise! No fog at all. The C41 seems to have survived best of all in terms of film speed, the TMax 100 and FP4 lost a bit of ISO/EI speed but will shoot well at say 80 or even 64 given what light conditions I am dealing with.
So in the summing up, the ONLY way to safely determine what is what is to shoot a roll and see.
As for buying expired film, well, you pays your money and you takes your chances. Ebay sellers for the most part tend to be non-photographers who seem to place ridiculously high values on everything they buy (or else live in perpetual hope of that big sale). I do buy now and then if something mouth-wateringly tempting comes up, like Kodak Panatomic-X (which nowadays seems to be worth its weight in pure gold), but I try to avoid auctions and look for Buy It Now sales or, best of all, those asking for Best Offer. I always pay reasonable money and so far have not lost any purchases by too low prices.
Now to find the time to shoot all the stock I have in my film freezer. Maybe if I live to 107...