Just thought I'd share my experience about a cheap focal lens I ordered from China via eBay. I decided to try the eBay lens out because it cost about 85% less than from a local dealer and I was not in a hurry to get a new focal lens.
I placed the order for a "New HQ ZnSe Focal Lens for CO2 Laser Cutting Engraving Diam 18mm FL 50.8mm" on 4. september. I decided to go with the lens that said "Origin USA" and pay a bit of extra (bout 50$) compared to the one that didn't say the origin so precicely. The seller was called win-win-mechatronic. They sent a message the very next day that the item had been shipped. I received the lens yesterday, October 1. so the shipping took almost a month. Was getting a bit worried, but thankfully everything went ok with the delivery.
The lens was wrapped in a protective tissue and placed insice a cushioned plastic case. The case was then enclosed in a bubble wrap envelope.
The original lens on the Epilog machine is 1 mm larger in diameter than the one I ordered, and this was because they only sold 19.5mm (instead of 19mm) lenses and I was not sure if the tiny bit larger lens would fit in the case, so I ordered a smaller one instead. The 18mm fits in nicely, although you have to carefully align it to the center of the chase before tightening the screws. I'm thinking of placing something around the lens (or the lens chase) to keep it in the middle more easily.
Then to the cutting results:
I had noticed a bit of a power loss since my old settings weren't quite making it through the materials I had cut before with ease. This was probably because my focal lens had lost the surface coating and had a few scratches because of inappropriate cleaning techniques (my mistake I won't be doing again). When I placed the cheap focal lens on, my old settings started to cut through again. I've only done a few cut tests, but so far it seems that the cheap eBay lens gives as good of a cutting result, as the machine used to have with a non scratched original lens.
Hopefully this experience is of use for someone!