The biggest problem with the traditional "comp check", from a USPAP perspective, is that there is typically an "if, then" aspect to it.
"If you can get 200K, then I'll send you the appraisal request."
That violates USPAP because it creates a contingency and the occurance of a subsequent event dependant upon a known desired outcome.
To be USPAP compliant, the client would have to simply ask you to provide an estimate of value on the property. Only the client can know what is required to continue to the next step. The request for a comp check can not come with an "estimated value" or anything similar.
That's my opinion anyhow.