Before leaving a negative review, you should exhaust the sellers complaints procedures. Even if you tried to use the CRA as it is now more than six months since you paid for the gas, the CRA would expect you (The customer) to be able to furnish sufficient evidence that the cylinder was faulty leading to the loss of the gas. At three years I doubt you could obtain evidence a leak being present at the time of purchase.
I seriously doubt the seller was deliberately trying to sell underweight or faulty cylinders refills, as there would be no commercial gain, because the retailer would be buying from a commercial supplier like for example Calor for Flow gas etc.
How could you prove you hadn't opened the gas valve at some point and possibly not fully closed it?
I strongly suspect that you would spend more money and time trying to make a claim against the seller, than the value the gas. and along with the strong possibility of not even being successful with such a claim, you could be landed with the sellers legal costs also.
Probably better for finance and valour to post the experience as "That's Life" And if you feel so strongly about it, double up on checking cylinders for weight in the shop when you first get them.