Applicant has existing mortgage, Reason for denial?

[Tom W.]

I have a applicant who makes good money, owned home for past 15 years with existing mortgage. Supposedly in process of selling home, but its not listed and when I ask for documentation to show its for sale or under contract, he wouldn't provide anything or further explanation. With no proof that he's selling home I have to look at the mortgage/prop taxes/insurance when considering his ability to cover rent. Also, I've had tenants in the past who leased property for a relative or used the property for airbnb.

I'm leaning towards declining app. He has good credit with solid employment and income, but the early uncooperation seems like a red flag. Is this a legit reason to decline app and what's a good way to word it? Just got done dealing with a lawsuit last year and don't want to get into another mess.

Thanks