I thought these might be pirated or fake
They are often legitimate product keys, but they will be sold without a valid license for their use.
Where these keys are garnered from is a matter for debate - officially it is against MS terms and conditions to release these keys, so it's probably illegal and MS may reject the keys when they are discovered to be in use.
For example, if a PC manufacturer had a volume license for preinstalling office on a number of machines, but not all the product keys were issued - 'Someone' may sell these keys on, against MS terms, for their own profit.
These keys exist, have been paid for, but haven't been used - the selling on of these keys could be seen as being in a grey area.
A clue may also come from the office version offered - i.e. Office Pro Plus is only available to organisations, via a volume license (It isn't sold for retail) - some organisations have a huge number of available activations - would anyone notice if a few went missing?
Or, if an organisation moved onto a new version of office, all the licenses for the previous version would no longer be in use - could these keys then be legitimately sold on?
The keys could also be stolen, pirated, fake, don't work, or are purely a scam; and if they do work, if MS get their act together, the keys could be annulled at any time.
It's always best to buy and download software from a reputable supplier.
Or, go for the open source options such as Open Office, or Libre Office.
but it is being sold on multiple platofrms including Very.co.uk
The products sold on Very are legit...