Think of it like buying a used car versus a new car.  You go to a dealer and tell 'em you want a new car with a certain feature set and they'll get it for you.  If they can't come close enough with their own inventory they'll get it from another dealer or order it from the manufacturer.  

Now go to a used car dealer and tell 'em you want a certain car with certain features and if they don't have one, you're SOL.  You can only buy the ones they have.  Come back next week and they'll have different stuff.

I think blems are similar.  I doubt they'd guarantee specs as they can only sell you the specific balls they have on hand.  There's no telling when or if a ball with your requested specs will even be made.  A good seller will tell you what balls are available and you could choose the one closest to what you want, but for seconds, where there's not necessarily much money to be made, they might not even be willing to do that.

SH