While it's "just a product key", the likelihood of it working long-term (potentially even short-term) is minimal.
You'll find countless threads on this very site of 'cheap' keys going wrong. Volume licenses, stolen credit cards etc are all potential sources for those keys - and run the very high risk of being deactivated, or failing to activate outright.
Whether they offer a replacement guarantee or not is a moot point - marketplaces allowing the sale of such licenses in the first place, are hardly likely to be the most reliable/trustworthy themselves.
Given you can run W10 unactivated with minimal restrictions, even $8 seems like a waste.