Add to Ozzie46, material. Brass, bronze, aluminum, cast Iron, nylon, delrin, steel, plastic and I probably missed a couple.
Case in point, I need two NA90 and one NA22B gears for a Steam Donkey. You need to actually look up the Boston Gear specifications to find that these gears come in two different shaft sizes, so you can't just Google the part number. So far there is a $60 price spread between vendors.
For example, NA90 specifications: 14.5 Degree Pressure Angle, 3/8 Face, 90 Teeth, 4-1/2 PCD 1/2" Bore, Cast Iron.
In your post above you want 1" aluminum spur gears. Is that outside diameter Or PCD diameter?