I don't know your pool guy so don't be offended by my comments. One word of caution, use a licensed contractor for any work like installing a new heater. Does your pool guy carry liability insurance? Did you know that if someone works on your house and is not licensed and their work causes damage and you try to go thru your insurance company they could deny your claim and also cancel your policy? It has happened so check your policy before you allow any person to do work on your property. What about if he gets hurt during the install? If he's not properly licensed and insured, he could sue you.
If he installed parts and the unit still didn't work, tell him to remove the parts. You should owe nothing. He can return the parts to where ever he purchased them and get a refund. Yes he could also install the parts and the heater works for another day and then it's another part that is bad. It happens and is the chance anyone takes.
He is not buying direct from the manufacturer unless he has a wholesaler agreement with them. Manufacturers sell to distribution houses and they sell to the end person like him. Build quality? Haha, that's a good one. I wonder how many people fall for that. I very rarely will install a unit someone else buys. The reason is if I install it and it doesn't work the homeowner could say I broke it during the install. Now I will have to argue with the owner. Now if they purchase thru me and it doesn't work after the install, I can take it back to my supplier and get another unit. It would be a pain in the butt to get a refund/exchange thru an internet place not to mention the time it would take. Also check their return policy.
As for the efficiency of a pool heater, be very careful of these claims. Have you ever priced out a high efficiency gas furnace for your house and compared the price to a regular unit? Considerable price difference. However, gas pool heater prices all seem to be along the same price. Wouldn't you think a higher efficiency unit would cost more? Also consider your cost of ownership for the unit. What kind of parts are inside and how much would it cost to fix down the road when it breaks. Just like a car, the more parts the greater the repair cost.
Just remember one thing, use a licensed contractor and protect yourself and your family. It could be a costly problem should something go wrong.