Office-2019-professional-plus-cd-key being sold for $62.48. Is this legal?

Professional Plus can only be sold by official MS "Partner" retailers.  The real retail price is about 10 times that price.

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They are reselling "volume" license keys contrary to the license terms. aka "illegally". 

When MS 'wakes up' and notices the company, they can block the product keys so that they no longer work.  You are left with non working applications. You have to decide if it is worth the risk to pay $63 for what may be a short term license.

If you want to buy a legitimate Pro Plus license, you have to find an official partner and buy a minimum of 5 MS licenses (of any type) including Pro plus to start your "volume license agreement".  At one time the list of software included a $5 Mapping product, so you could pay a $20 "surcharge" to get a legitimate license.

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Search MS Partner Network

https://partner.microsoft.com/en-us/support

https://www.microsoft.com/en-ca/microsoft-365/business/microsoft-office-volume-licensing-suites-comparison (2019 Standard, Pro Plus, Standard for Mac)

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PS MS has quietly announced that they will be releasing the next onetime payment Office version, "Office 2022?" "sometime this year".   As a long term cost saving strategy you may want to consider getting a 365 monthly, or even year subscription until the next version is released.  Office 2019 has a 7 year "end of support" life, ie it ends October 2025.  Office 2022 will probably have an end of life of 2028, 3 more years.  On an expensive bundle like Pro Plus spending $50-$70 for short term 365 makes "dollars and sense"<sic> to me.