Doug,
The '71 SX convertible is a rare and very desirable car.
Pictures, or a link to the ad will be helpful.
The vin# will determine conclusively whether it's a Cutlass or a 442 (but not that it's an SX).
1) If it's not advertised as an SX, what criteria are you using to determine that it is?
2) Are the engine and trans original, numbers matching to the car?
3) Does it have the dual cut-out rear bumper?
4) Is the owner advertising it as a 442 "tribute"?
5) Does the owner know anything about the history?
If it's a 442 tribute, the 442 badging, etc. can be undone and reversed.
If the car is a confirmed '71 SX Convertible, then I believe the asking price is reasonable in the condition you describe.
Kevin