This original 300mm 2.8 was a great lens. It is hands down the lens I most regret selling. Actually it's the ONLY lens I regret selling. Not that any of the other lenses I've sold or traded over these many years were bad, just that their replacements were as good or better.
But this lens was special. If you like to manual focus like I do, this lens has the best "feel" of any Canon lens I have used. There is the drawback that even manual focusing sucks on the battery, coupled with the poor guarantee of available repair parts, but the touch of this lens is awesome. Being confined by finances to the used market for this type of lens, I would seriously consider purchasing a copy of this lens even knowing it may take a second copy for donor parts to fix later.
My opinion is that the optical superiority, if any, of the first IS version over this lens is minimal, and the removable tripod collar on the IS version becomes gritty and stiff over time (based on at least two copies I have rented) and can allow the lens to dislodge from the collar and hit the ground at an inopportune time. My 300mm f/4.0 worked loose as I was carrying it on a monopod over my shoulder once, and it was sheer luck that it happened in tall, soft grass and not on the hard gravel I had been on just ten steps before. The original version 300mm 2.8 has a secure, permanent collar that stayed smooth the entire 12 years or so I used it.
An earlier poster commented that the "electronics are not likely to last much longer." I don't know that this is true. These lenses are still fairly common on the used market and in use in the field, and I don't recall ever hearing of any electronic failure that did not result from a drop or other damage. Assuming you get one in good condition, I'd expect years of value out of it, especially if it's not in everyday use.
As for the specific copy referenced by the original poster, definitely agree with others who have cautioned to examine it very carefully for proper function and good condition and ensure non-problematic provenance (be sure it's not hot), but I can't say enough good about this lens in general. The price does seem a bit too good to be true, but in my opinion, the price of this lens should be moderated by the lack of repair parts availability.