Take a small flashlight with you, to inspect inside the lens and camera. Download the camera manual and read it in advance, to be sure you know how to operate it and can quickly test it thoroughly. Maybe even take a laptop, to look at some test images more closely than is possible on the camera's LCD screen.
Assuming it's a Craigslist or similar sale... yes! Meet in public! Safest is a busy bank lobby or inside a coffee shop, or similar. Most of those sales are cash only, and it's a bit risky meeting up with someone who knows you have $2000 in your pocket. There have been numerous Craigslist-related robberies, with large ticket items such as camera gear used to lure in the victims.
The price is good... and not necessarily too good to be true. A lot of time people sell things below the going price, to get quick cash. I've gotten some great bargains off Craigslist ($1000+ set of wheels & tires for my bicycle only cost me $350 off CL, not too long ago... I bought 300/4 IS off CL too, though it wasn't at as much savings as that).
If it is someone wanting to make a quick sale, they might not be willing to wait for you to sell off your gear and gather the money, if that's a consideration.
Only you can say if this is a good deal for you. Selling off your present gear to get the FF camera with just the one lens, you'll have half the telephoto "reach" you have with your 28-135 now, on a crop camera (135mm on crop is equal to roughly 216mm on FF). But you'll gain a wider angle of view (24mm vs equiv. to about 45mm now, after converting 28mm on crop to FF equivalent). And, your 50/1.8 is over two stops faster than the f4 lens. Hate to see you sell the flash... the 5DII doesn't have a built-in (not that built-in flashes are worth a damn). You'll be going from 18MP to 22MP... yes, FF will have a bit more fine detail/resolution, and it will be good for about one stop higher ISO. But the 5DII likely won't be as good tracking moving subjects in AI Servo, as your present camera... if you take a lot of action shots. (I don't have T2i, but do have 5DII and I don't use it for action photography.)
Don't expect to see much different image quality from 24-105, than you can get from the 28-135 you have now. It's just not all that different... the L has just slightly higher resolving power. The 28-135 tends to be a little soft at 135mm, but the 24-105 tends to vignette at the 24mm end. The 24-105 has slighlty less pincushion at the wide end, but more barrel distortion at 105mm. In terms of IQ alone, it's sort of a wash. You won't see faster or much closer focusing, either. The build quality and sealing, non-variable aperture that renders a little nicer bokeh, slightly better IS, and probably longer-term durability of the L are what sets it apart.