Bring a fresh battery (group 75, side post) when you do a test drive. Sometimes, certain vague conditions specified by a seller, like that, or "bad fuel pump", "carb needs adjustment" just mean it won't run, and they are minimizing whatever condition(s) actually exist, or they don't know what is actually wrong. Anyone not frail, who is marketing a vehicle, would have the battery charged and ready for test drives? Do not assume any car is rust free, even if they say "always garaged", "never driven in winter", especially a Fiero. Check the underside, engine cradle, and under the trunk carpet.
Fiero exhaust manifold sometimes crack, this forum will provide photos of problem areas to check, but seeing a manifold crack can be difficult while it is on the car. Replacing a cracked manifold can be expensive at a shop, or a struggle to do yourself, so get that diagnosed for sure before negotiating a final price.
This forum is a great resource. You can find answers and descriptions for all the good and bad aspects of Fiero ownership here, just ask, or use the forum Search function.
1988 Pontiac Fiero Formula, Yellow, original. (CJB #118)
1977 Pontiac Le Mans Can-Am W72, original, unrestored.
1977 Pontiac Grand LeMans
2003 Pontiac Grand Prix GTP L67/MN7 (Parts Car)
2001 Pontiac Grand Prix GTP L67/MN7 (DD)