Authorized Re-sellers

Hello TonyWilkerson

Short answer: You don't/can't.

The rule to follow is "if it sounds too good to be true - it generally is".

To put this another way, Microsoft isn't in the business of giving their software away. Best practice to follow is to stick with resellers that you already know and trust. You will never find "heavily" discounted products from any major resellers.

What you'll find all too frequently is that resellers promoting Office products at insanely low prices are providing Volume licenses that are not legitimate retail products. The license key provided may work the first time you use it but may also suddenly find one day that your product is no longer licensed with no recourse other then to go back to the original vendor which seldom (if ever) ends well.

Carl has summarized the answer fully.

By dirt cheap do you mean around US$50-70 for an Office onetime license. Yes, that is too good to be true.

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You can't even trust sellers on Amazon!  Amazon says they don't allow bootleg products, but that is mostly talk. Amazon is more interested in taking their cut than verifying legitimate vendors.

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On check you can make is find out how old the website is. If it is less than a year, suspect it.  As Karl said, stick with long term, well established vendors. Vendors that have a presence in the real world.

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Another kinda "scam", is selling OEM licenses to end users. The license terms of OEM say that type of license can only be used by a computer builder who sells the computer to an end user. Not by anyone who will be using the computer themselves.

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I had an interesting discussion about EU law. The other person was of opinion that original buyer of the license could assign it to someone else, without installing it. I'm not convinced.

on 08-01-2020 I bought office 2019 professional plus through indiduval    seller through online portal less than 10 dollars.

key activated through online Microsoft server.

instead of resellers, you can directly buy the genuine key from Microsoft store with your own Microsoft account, but you need  CREDIT card for buying

Another kinda "scam", is selling OEM licenses to end users. The license terms of OEM say that type of license can only be used by a computer builder who sells the computer to an end user. Not by anyone who will be using the computer themselves. 

I had an interesting discussion about EU law. The other person was of opinion that original buyer of the license could assign it to someone else, without installing it. I'm not convinced.

Rohn,  If I understand the above correctly you do well not to be convinced.  OEM licenses can only be installed on a computer by the authorised computer builder.  As you say selling such licenses is an infringement of Microsoft's terms of licence.  Once the software for Office is installed on a computer it becomes the licensed computer, it can only be transferred to a third party along with the computer.  Just a few years ago there was a scam whereby the original buyer/computer builder of the licenses for Office sold them along with some small bit of hardware claiming that made it legal to install Office. Microsoft did catch up with the person and he was convicted, not sure what sentence he received. EU law is no different other that for Germany whereby the license is subject to German law, not sure what difference that makes.