Wow. If you sold a graded one of these for $750 then you must have been wearing a mask and a stripy jumper at the time.
Personally, I would disregard the grading here as it is so low, and you can get a nice acrylic case for the Meccano cards off Christian for circa £10.
I would say the market value for this is approx £100 but I personally wouldn't pay that due to the bubble crack so maybe nearer £70-£80
I had a cheeky low ball bid on one of these in the vectis auction. Ungraded but good nick it sold for £100 and I know toytoni was selling his for £110 on ebay, although it seems to have gone up to £120 now, if you can bear to give the piece of **** another penny piece.
Just my opinion on the matter. I'm no expert. Sorry to disappoint you here but since you asked. Good luck with the sale by the way.