Welcome to the forum.
I'm assuming it's a 4-stroke engine given its 2010. A 2015 4-stroke Mariner short-shaft costs roughly £1,600.
A useful rule of thumb is you lose 20% of the value in year one, then 10% every year there-after. I reckon £850 to £1k would be reasonable. Now is a good time to sell given its the start of the season.
Unfortunately low hours means very little on a leisure engine as there's no way to determine that, unless there's an hour meter, etc. Better to have regular hours and documented service history.
Some 8hp engines here for reference: http://www.seamarknunn.co.uk/download/usedob/used.pdf