Purchasing Windows 10 queries

noob question,

anyone knows if i purchase windows 10 DVD from retail,
can i download the version off the Windows website and use the license key purchased from the retailer?

saw some license selling dirt cheap on shopee/q10 but remain skeptical

reason is i'm trying to set up a new rig but do not wish to purchase a USB dvd reader.

trying to create a boot drive from USB.

any other advice would really be appreciated.

Edit: just realised posted in wrong forum, pls delete if deem fit, will create another thread

TIA