Thanks for all the responses!
They made an effort in their letter to sound "personnal" and sort of mom-and-pop. Based on what you guys are telling me, this is not uncommon but it's no reason to believe they are malevolent. It's a question of whether or not we feel like selling...
Although their comps are actually accurate, their main offer is below the amount that we paid for the house. Our plan was to wait another 2 years or so for prices to go up a little more.
They had a secondary offer where they would pay us 10% higher than what we got the place for, but it would be with monthly payments for 60 months (followed by a lump payment in the fifth year).
Basically, they think we might be "distressed" sellers hoping for a way to get out of the house now instead of later.