I wont buy anything now I suspect to be a 'knock-off' because of the damage they are doing to the astronomy equipment market.
I know they are 'affordable', sometimes insanely so, but buying them means the designer and/or brand name is losing money.
Astronomical eyepieces are precision components, they are not that easy to copy or knockoff properly. It takes some skill and knowhow to design an eyepiece, so making a cheap copy of a cheap eyepiece is a waste of time, instead it is simpler to use the same components, but skimp on quality control. For expensive eyepieces, the customers are generally knowledgable and quickly find out if the product is inferior, so you can't play that game either.
TL;DR: if a name-brand eyepiece is available much cheaper than normal, there are some simple explanations:
eg Televue Ethos eyepiece for $99.
2. CLOSING DOWN SALE
eg Vixen's US distributor recently held a liquidation sale.
3. NON-EXCLUSIVE DESIGN
eg Markus Ludes got someone to design the APM eyepieces but did not secure exclusive rights, so the factory was free to sell the same eyepieces to any other buyer. A buyer willing to order in larger quantities would pay less per unit and could in turn resell the eyepieces at a lower price. Another (more controversial) example is the TMB 58-degree eyepieces designed by the late Thomas M. Beck, he did not secure exclusive rights either and these eyepieces can be bought under various brands.
Exclusive design example: Kevin Busarow of Oberwerk (OB) owns a stake in one or more binocular factories, so he has exclusive rights to some designs in the US.
4. LIMITED SUPPORT
AliExpress vendors typically have a 75-day return policy, but nothing beyond that. In contrast, many local vendors offer 1-year or 2-year warranties, or even lifetime warranties. Warranty support costs time and money. Even though the brand principal is the one who repairs/replaces the item, the vendors have to handle the returns etc.
This price difference is especially obvious with ES eyepieces on AliExpress - numerous reviews indicate the eyepieces are genuine. Fixed focal length eyepieces have no moving parts, so if they work fine on arrival, they should work fine for many years. Such customers sacrifice the longer warranty to save money upfront.
5. LOWER MARGIN
Every brandowner and vendor is free to decide what margin they want to earn. Even in the US, this is obvious when comparing Orion with Meade, Celestron etc. In some cases, the same eyepieces (in different clothing) cost significantly more under the Orion brand. That does not make the Meade or Celestron eyepieces fakes or knockoffs. Likewise, a Chinese ES vendor who expects to sell 1,000 ES eyepieces a year might accept a lower margin per eyepiece than an American ES vendor who expects to sell 100 ES eyepieces a year.
Depending on the country of the purchaser, import and sales taxes vary a lot. In the US the import tax is currently 25%. Some CN members have reported that in their country, import taxes can reach 300% on telescopes, so they choose to import the optical components and build the telescope themselves. An item may genuinely be cheaper on AliExpress, but after adding the import and sales taxes, there may not be significant savings.
Some people try to evade import taxes by having vendors under-declare the value of the items. That is another issue altogether.