"The dealer reset the check engine light before I got there and says it's just a sensor." Is this legit?

Be careful. "It's just a sensor" is one of the most useful sentences a seller has, because it makes a warning light sound like a formality, and clearing the code right before you arrive buys time before the light comes back. The check engine light comes on when the car's computer logs a fault. Clearing the code erases the record and turns the light off, but it does nothing to the underlying problem. On most cars the light stays off only until the fault repeats, which can be minutes or a few drive cycles, which is just long enough to get through a test drive.

There are two things to check. First, plug in a scan tool, or have one ready, and look at the readiness monitors. After a code clear, the car's self tests reset to not ready and take dozens of miles of varied driving to complete. A car that supposedly has no issues but shows a wall of incomplete monitors had its computer reset recently. A disconnected battery can do that too, but on a car a seller cleared right before showing it, a wiped code is the likely reason, and the question is what was wiped. Second, look for stored freeze frame data and pending codes, which can survive or reappear and point at what the cleared light was actually for.

The deeper concern is what hides behind a cleared light. Sellers clear codes to mask a misfire, a failing catalytic converter, an emissions fault, or a transmission problem, any of which can be expensive. A genuine just a sensor is cheap to prove: a seller who believes it will let you take the car to a shop for a scan, or hand you the actual code so you can look it up. A seller who cleared it right before you came and will not let it be rechecked is hoping the light stays off until the car is yours.

If the monitors are incomplete or the light returns on your drive, treat it as an open fault and price it as one, or walk. The cost of the repair is unknown until the code is read, and that is the seller's reason for wiping it.