Its kinda hit and miss, a lot of people who have the singular are not interested in the plural and a lot of people with the plural are not interested in the singular.
I know when I registered MapleDots.com & Mapledots.ca way back in 2011 I could easily have registered the MapleDot version.
Fact is I was not interested in the singular because it does not make sense to my business motto and even if I was offered it for 10 bucks I would not buy it because I have no use for it.
My advice...
If you already own the domain, go ahead you have nothing to lose unless it is a trademark.
If you're going to spend money purchasing a plural with the intention of selling it to the singular owner it's best to do research first because not everyone will be interested.
The are a number of topics on the forum where a member is trying to sell EFTYS.com to EFTY.com
The owner of EFTY clearly states many times he is not interested and the member keeps trying to convince him he should.
Technically the definition of a plural can be far different than the singular and vice versa.
Can you imagine an Apples computer?