If the sticks and buttons are for you, its probably fair deal.
Personally I find it's hard to think of a supergun as much more than a power supply and bunch of wires, but if you don't want to make one yourself, and the controls are fine for you, the build quality is decent - its probably a fair deal if you can pick it up without added shipping.
Cautions [room for negotiating the price?] :
You might want to check out what you'll need to do if you want to harness all the buttons.
Looks like it comes with composite av cable (?) ... might want to check that you can get a rgb cable and/or get pintouts - although I guess you could trial and error to make a new cable.
Check what the speaker/headphone output is like.
Can't comment on specifics, but I assume it comes with an adjustable arcade powers supply, which can be nice to have for picky games [again may depend what boards you're interested in using but its nice to have anyway I suppose].
Those Taito Egret II candy cabs are awesome for their ease of tate'ing the 29" dual-sync Nanao arcade monitor. It's been said that you can actually rotate the monitor with just one hand. Plus the cool ability to hook up stereo output arcade PCBs like the MVS MV-1 motherboard, Taito G-Net, Sega Titan ST-V mobo, etc. Playing gaming consoles on it via a Scart to Jamma or Ultracade UVC device is a breeze on an Egret II provided that the Nanao monitor hasn't been upgraded to a tri-sync monitor (i.e. Wei-Ya tri-sync monitor chassis upgrade).