Definitely not a legitimate license at that price. Probably technet keys which will be blacklisted and deactivate in a couple months.
You can use Windows 10 for free if you want. Personalization options are disabled in Settings and there is a watermark on the desktop. If you can live with that. Then you can just download the ISO from MS and make an installer DVD or USB. That's the method MS provides for people who can't or won't buy a license.
If you want a legitimate license. It's going to run $90 to $100 on PCParticker.com for Windows 10 Home and $120 to $130 for Windows 10 Pro. That is the OEM versions. Retail licenses run more.
Hello I am picking the parts for my first PC and I looked that on eBay they sell windows 10 for 10 dollars. Should I trust that? Where I can find Windows 10 cheap?
In no way is that legit. Multiple ways for you to waste $10 and gain a lot of headache.
Just install it, and leave it unactivated. $0.
It runs exactly as a real purchased license, except for a few small restrictions as mentioned above.
You're building a whole new PC?
In your parts search, if you saw 16GB DDR4 RAM on ebay for $20, you'd run away laughing, knowing it was a scam.
Licensed software is no different. It costs what it costs.