I too bought mine on eBay. After receiving it I did the salt test calibration. Was unable to get the one I purchased, to calibrate. The adjustment screw would not turn, and was 17% off out of the package.
Would have been a great item if able to calibrate, was well made, easy to read scale but not calibrateable, is the reason I give a 1 star rating.
I bought on Ebay for $6.99 and was not expecting much. When it arrived I was impressed it is brass with a glass face. When I checked the calibation it was only 1% low it read 74% instead of 75%. After using it for three hatches it is still just 1% off. For what it is worth the seller says it is guaranteed to last for life.