I don't know about the ebay adapter since i don't have one, but in
this video they show that they solder a wire to a capacitor instead of a transistor.
If I remember correctly (I don't know where I got this info from) the wire to the cap, was to stabilize the current that went to the back light.
Since according to the seller, there were some GB/GBC and apparently some GBA games that used more power thus making the back light or something unstable.
I personally haven't experienced any problems with the original adapter, I tried out two of the games shown
in the video (Castlevania & pkmn silver) and I didn't experience any issues at all.
The only user that recently posted any issues with GBC games was
driverdis, but he solved the problem by swapping motherboards, since the one he received with the mod preinstalled seemed to have some acidic damages to it.
Also if you don't solder the wire to the original adapter you get an almost unplayable screen.
Like this.