If it's a copy or "clone" that say Aimpoint, EoTech, Trijicon, etc. then it's illegal to sell, and techynically illegal to buy and own.
I am not playing netcop, but imagine for a moment you advertise your rifle with "clone" optic, somebody wants to buy it, you meet up with them and get the rifle confiscated and possibly arrested or your pistol license yanked for selling counterfeit merchandise. Is it worth it because it said "EoTech" on the side instead of "Xinshao"?
Another issue (and one that is not your problem from a legal standpoint but warrants discussion from a philosophical/moral standpoint) is that some nations (read: China) don't honor patents and trademarks. So, they can basically rip off EoTech or Trijicon's technology and the real companies can't do anything about it. You can then get a similar, more cheaply made copy of the original for much less. A good example of this is NCStar.
Then, there are the "Airsoft" sights which are the bottom of the barrel, and my personal opinion is that EoTech and the others should just make these in the same crap factories in China and stamp "For Airsoft use Only" on the side and sell them themselves since it's a huge market, or license the trademark.
There are differences in the technology between holographic sights and LED reflex sights. There are difference in the quality of the optics, ruggedness, waterproofing, etc. No, the $30 Aimpoint clone marked up to $75 on eBay is not filled with pressurized nitrogen to resist fogging and you can't do your 40' underwater rebreather swim with your SEAL team unless you can shoot while looking through an aquarium. But for most of us and the way we shoot, so what?
Most of us need good enough optics to transmit enough light so we can see the reticle on an overcast day or dusk, we are not doing night recon missions. We need a reticle we can put on the target, but whether the dots are calibrated to the caliber of round we are shooting is not such a big deal. And seriously, most of us have at least a few cheap guns like Hi-Points, who in their right mind would put a $600 optic on a $250 gun?
Now, some of us also have "real deal" guns, if you have $2K-$3K into your AR and want a quality optic, go buy the real thing, you don't want some $50 POS on there anyway.
The best inexpensive optic I have found to date is a reflex sight, available from Aim Sports (aimsportsinc.com) the item number is RT4-03 and I paid about forty bucks. It has red plus green, 3 brightness levels on each, and a lever that changes between four reticles. Best of all, if I go to sell it i don't have to worry about getting arrested, or some guy getting PO'd because he thought he was getting the "real deal".
Buy- and especially sell- the "clones" at your own peril. The penalties are potentialy VERY steep:
CHAPTER 113--STOLEN PROPERTY
Sec. 2320. Trafficking in counterfeit goods or services
(a) Whoever intentionally traffics or attempts to traffic in goods
or services and knowingly uses a counterfeit mark on or in connection
with such goods or services shall, if an individual, be fined not more
than $2,000,000 or imprisoned not more than 10 years, or both, and