I don't do this as a business, more like a hobby, so I'm sure my understanding is limited only to what I personally do.
From what I understand- in a situation like repairing a computer and reinstalling an OS, the buyer should have a retail copy of the OS, which is a license that can be transfered. (If that's what you mean by unlimited). There are volume licenses as well, but I think these are for companies installing in bulk.) I would guess there may be a way to get retail licenses in bulk for a discount if you were really doing a ton of business, but I don't know for sure.
Personally, I've never had an issue using an OEM license on a system I've repaired/reinstalled for someone. Even though this type of license says its for new computer builds only, I've never run into the slightest issue using them with a computer someone already owns. I also as I said, don't sell Windows for a profit, just what it cost me, and of course the price of my services doing an install.