Other criteria are that the house must be suitable for occupancy. You can't buy a house that has major repair work needed, unless you get a larger loan with an escrow repair or have cash to repair it and the lender consents to that.
The property must be insurable and not something built in an area that geographically might jeopardize the property.
The house must not be in a flood zone that cannot be covered by flood insurance.
The house must be have a certificate of occupancy and be zoned for residential purposes. You can't buy a house to live in that is zoned for commercial use.
These are the obvious ones, but other matters can interfere in a purchase.
This question sounds like a buyer or their Realtor is telling you that they cannot purchase your property for the above reason.
I do not know the market in your area, so let's just say that on average, most housing markets have declining values right now. Appraisers and underwriters are under the gun to make sure that their work is squeaky clean, some having a property appraisal not meet the sale price simply means that the property is not worth the sale price.
There is no law that prevents people from paying too much for property. However, banks will NOT lend more than the appraised value. So if you need to sell, ask some questions of the buyer or their Realtor so you can figure out how to get your home sold.
It sounds to me that you are probably selling without professional representation, so if this is the case. do you think asking a bunch of professionals who know nothing about your property or this transaction can really provide you with competent help?
If the home is not in move in quality an appraiser will note that on report published furnished to bank. If house has roof or foundation issues are only a few issues lender could deny approval for loan. Keep in mind bank wants to loan you money however they are also at risk. If a bank declines a loan you should take that a strong statement to you reconsider not purchasing that property.
Appraisal is one....
Perhaps not built to code or with approvals....for example enclosed garage without permit or built over the building line on the survey.
Unseasonsed homes in some cases.....bank might not finance a flip.
Owner Occupancy Rules for Condos