I recently noticed a glut of reproduction pink Mayfair cookies on EBay masquerading as the real thing. So I am going to give some examples.
Real or Repro has a great article on this
This is an example of a lid that I know is a reproduction. I can tell immediately by the scrolling in the border. 
The authentic border looks like this:
Here is another example of a post I would beware of:
In this example you can see in the close-up that it too has a pronounced wavy scroll, instead of a flat one.
The knob also looks to be the hexagon shape that most reproductions have. The authentic jars have an octagon shape with 4 long edges and four shorter flat corners, which makes it much squarer than the hexagon on the repros that looks more round. Here is an authentic knob:
As a side note, the design on the authentic lids is delicate and well formed. Some reproductions have less well formed designs. In person the well formed designs feel rougher while the poorly formed ones feel smoother, without edges.
I would not buy this jar. And then there is the other huge red flag: there is no picture of the bottom of the jar. A real jar would have an obvious mold mark ( a circle on the bottom pressed on the glass) like this one has:
But this post has a big problem, the bottom might be real but the lid is a reproduction! It is common to break the lids so some sellers slap a reproduction lid on an old bottom and sell the whole thing as authentic.
This example is irksome to me especially because of the price. I feel as though it’s a scam. So if you want an authentic pink Mayfair watch out for all the fakes on eBay!