Can't really divide it as eBay brushes and high end brushes. Let's compare Simpsons vs eBay (though most of these will carry over to other manufacturers)
Handmade knots
Better hair sorting
Higher quality material for handles
Turned handles vs molded/injected plastic
With relatively few exceptions, denser. In most cases far denser. (There were guys saying their FS were denser than their Berkeley when FS fans were spamming these forums up last year. I measured it. My Berkeley had 3-5 TIMES the hair of any of the three FS I had.)
You may like Lijun or FS brushes better. But there are pretty obvious reasons why Simpson, Rooney, Kent, etc brushes cost more. And when people go around with statements about how they don't use their duke or their Rooney Super or their whatever because their eBay brush is so much denser, it makes taking any of their opinions seriously require a level of gullibility I lack.
I've tried three different eBay makers. TGN knots were superior to all three by a LARGE margin. And TGN knots are direct price competitors with all but they cheapest of them.
Evening mate. Read your post last week, I thought I'd give my take on it after a few uses of the Lijun.
Well four shaves with the Lijun 'Finest' 24mm has left me more than pleasantly surprised.
I've owned two TGN Finest brushes in the past, one extra stuffed and the other regular. In my opinion, the Lijun is hands down better than both knots offered by TGN. The hair seems firmer, yet softer. And the Lijun knot is closer to the extra stuffed offering than the regular. In fact, I think you can see from the pics I attached above, the Lijun is one well packed beast.
Yes it's not got a hand turned handle. I accept that. But so far, I can not fault any of the workmanship. I've not lost a single brush hair so far, and that's something even my cherished butterscotch Duke 3 can't claim.
I can't compare it to a 'FS' brush as I've not used one. And seeing as the Lijun offering was half the price of a Frank Shaving Brush, I doubt I will.
Honestly, for £12 I'd struggle to find better bang for your buck. It may not be my favourite brush, but it's up there.