The white '84 was for sale mid 2005 when I was looking, so I think the ad is likely now out of date. At that same time the Red '83 was for sale in London, ON at $55K, so I think these two cars were comparably priced at that point.
I believe prices have moved upward substantially since then, so I think you likely need to make a comparison to what is available and on the market now.
PI has a pristine example ’87 GT5 for sale at $80K
PI sold a Yellow GT5 conversion earlier this year that was advertised at $62K
There was a GT5 at John Scotti Automotive, Toronto late last year for CAD$79,500.
So in comparison an 83 GT5 priced at $65K looks a good buy, plus you have the added advantage it is already in Canada and a private sale (No GST?). I think late Canadian cars will always fetch a premium in Canada over similar US cars simply for the ‘no hassle’ factor associated with importation.
If I was looking for a late car I think the GT5-S represents a good buy at the moment, PI has two such vehicles advertised at present;87 GT5-S
at $83K83 GT5-S conversion
The latter I saw on display at the POCA Las Vegas track event, when it was $100K, at the reduced price I would consider a steal, providing you have no smog laws to deal with.