Too good to be true
Definition 1 of 2
Pro Tip 1/3Realistic Skepticism
Use when an offer or situation appears improbably favorable and you doubt its authenticity or viability. 
A job paying that much with no experience required? It's too good to be true.
Pro Tip 2/3Hidden Problems
Implies that potential issues or scams may be masked by an overly appealing facade. 
This car is so cheap and in perfect condition. It seems too good to be true.
Pro Tip 3/3Initial Excitement
Express initial enthusiasm but pair it with caution regarding the reality of the situation. 
They accepted my offer on the house without negotiating—it's too good to be true.