This is OLD bug and it is happening again.
Nothing to worry about.
I do not know what sellers are seeing but me as buyer I see “Last seen XX years”
This bug is happening again Last Seen 47 year ago?
First order:
Second:
wow ! how funny! 
You could report it them at the helpdesk.
It’s probably because the epoch date of Unix/Linux (Unix time - Wikipedia) is set at 1st Jan 1970 which is about 51.255 years ago.
So maybe the date they were last seen was not set/set to null and calculating the difference between that date and today gives 51 years.
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Yes, that sounds right because 47 was 2017.
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Fun video concerning bit overflow!
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