I don't consider it as a great deal. (It might be considered as such in Europe.) A great deal for a 1D Mark IV for me right now is somewhere around $2,800 or under. With the extras, maybe $3,000.
If you're worried that this might be a scam, it's probably not. A scam that I've seen on Craigslist normally offers something at a ridiculous price. Like a 1D Mark IV with 70-200mm f/2.8L IS II for $2,500. For this one, it's the price at a legit seller is likely looking for.
Regarding 5D Mark III vs 1D Mark IV, they mainly serve different purposes. I'll go with a 5D Mark III if you don't have anything specific in general or do mostly portrait and landscape. If you do wildlife or sport, you should go with the 1D Mark IV as it's still one of the best, if still not the best, Canon DSLRs in that category.