Well, since I also built a handful for myself, I can tell you that they definitely look different than the original usually in a few ways. The two that I did not need I sold clearly marked as "replica" and I do not pretend for them to be original.
My cards always have IC sockets, which the originals never had. So if you see mine being resold, they are easy to spot. Also the potentiometer probably looks a bit different, since I think the original one is not available anymore.
In the end, if it does not come in original packaging, or is obviously a version 1 card (with big headphone jack), I would nowadays assume that it's fake.
For me, I don't care about the AdLib being genuine. I just love the technology and the sound. That's why I am happy with a replica. So as long as you don't pay an excessive price for the replica, I think they are fine for use.
However, selling them as original is unethical. How much do people charge for the fakes? When I build them I pay about 60-70 EUR per card for the PCB and parts. Plus roughly a days work assembling and testing them. However I have built the ones I need, but I found it very entertaining and educating to do it. I learned a lot about soldering and sound cards.
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80486DX@33 MHz, 16 MiB RAM, Tseng ET4000 1 MiB, SnarkBarker & GUSar Lite, PC MIDI Card+X2+SC55+MT32, OSSC