Hi -

Test drove a '97 RodeoLS V6 3.2(?) 94K miles last night, overall fairly decent shape - but the heater starts to blow (very) cold air when the engine idles. Hot air is nice and toasty when vehicle is moving. Any ideas?

I sometimes have this problem with my Rodeo. Haven't really nailed down the exact cause yet, but I think its just slightly low coolant level. If it's down even a little bit, it can cause this.

it's lack of coolant flow, which could be lots of things, including low on.
also...
partially clogged radiator
partially clogged heater core
collapsing coolant hose
partial operation of thermostat
water pump

edit add: whatever, you will want to diagnose and fix it right away if you buy it. or get it diagnosed as a dicker point before you buy. start with checking the level as suggested
//bc