Puppy Face , Dec 20, 2005; 11:45 p.m.
That's the original and it's da bomb, well at least back in the day (1987). It only lasted a year before it was replaced by the MK II in 1988. Optically it's identical to the MK II but has some minor physical changes. As I recall it was an expensive optic in the late 80s, costing upwards of 400 calms at New York discounters. I couldn't afford one and had to make due with the lowly EF 35-70 3.5-4.5.
Although it's a rare zoom, it ain't worth too much anymore. Consider than an EF 28 -105 3.5-4.5 USM LL is only $225 new and pretty much kicks Ef 28-70 3.5-4.5 arse. So I personally wouldn't pay more than $50. Maybe $75 if it's mint and you want to do the guy a favor.