I think Danny just wants to take the time to make sure he's doing things as correctly as he can. That's what a university education and the principles of accounting drive into a person. I find I'm the same way whenever I start dabbling in anything I've never done before, such as when I started rebuilding my own laser printer toner cartridges. I wouldn't fault the guy for it, he's just wanting to partially compensate for not being formally instructed on how to rebuild that pump by using the best parts he can when rebuilding it. His only other option is to buy a "repair kit" for the pump, and that means paying $25 for $5 worth of O-rings. That's always the case cuz the company making the repair kit knows it's customers don't know anything about O-rings and would be scared they'd buy the wrong ones.

Danny:
Your best bet would be to take the O-rings out of your pump down to any place that sells O-rings to the hydraulic and pneumatic cylinder and valve repair shops in your area, tell them they're from a swimming pool pump and follow their advice. My guess is that they'll sell you the same 70 durometer nitrile O-rings that are what everything in plumbing uses, and you'll have everything you need for less than $5.

I have 66 faucets in my building; 21 kitchen faucets, 21 bathroom sink faucets, 21 Tub & Shower faucets and three laundry room sink faucets. I can't remember the last time I bought an O-ring in a hardware store or plumbing wholesaler. I buy all of my O-rings from Kepco Sealing Supplies in Winnipeg, and they primarily cater to the hydraulics and pnuematics industries, not to the plumbing industry. But all the plumbing hardware you buy in North America uses standard size O-rings, just like the hydraulics and pneumatics sector of the economy does, so they're completely interchangeable. The manufacture doesn't know which O-ring is going onto a kitchen faucet spout and which one is going into a pneumatic cylinder, he just makes them to the standard size out of the same rubber, and the O-rings he sells can be used equally well in either application.