Re: Win 10 OEM or retail?
An OEM license should be used on a new computer that you've built. From a compliance standpoint, the license isn't supposed to be moved to another computer. If you swap the motherboard it will likely trigger a deactiviation and you will have to activate Windows again. I don't think that's a big issue. Same thing will happen with Retail keys.
But with Retail, you have the option to completely remove the license from one machine and move it to another. You can use a retail key on an existing machine that you didn't build, but you're not supposed to use an OEM key to upgrade another manufacturer's PC.
Again, this is really all from a compliance standpoint as far as I know. If you just have two PCs for personal use or a friend/family member, I doubt you will have an issue. When you are building lots of computers for clients, then it becomes important you get this stuff right.
Bigger concern is buying license keys on ebay. I've heard stories of people who've done so and the license became deactivated later because the seller resold the same key multiple times. Make sure they come physically sealed in packaging IMO.