Does the $1400 include a monitor? The 15" LCDs are in the $300 range these days.
First let's take the $1400 933/superdrive against the new 1G powermac. The used machine has more memory and drives and a superdrive. In fact, it's maxed out. To make the new PM match these capabilities would go over $2K, and you still need a monitor. OTOH, the 933 is used and the drives may have some miles on them.
Against the iMac, getting the iMac to 1G RAM could be costly. And I don't think it'd be faster than the 933. But it does come with a screen.
So I'd say if the $1400 includes a 15" LCD or better, go for it. If it includes a 19" or bigger CRT, go for it. If it includes a smaller CRT or no monitor, try to talk the guy down to $1200 and shop for an LCD.