not sure why any seller would need you do give permision for a credit check. Proof of funds should be sufficient to show that you can complete the trasaction. IT does not happen very often but there could be a reqquirement in the Condo Doces for the Assn to approve the buyer. In such a case, the Assn my require it. So check out the Condo Docs before you refuse
You do not need a credit check but you do need to be able to prove where the money came from (bank statements, documents of sale, ...) this is a Federal requirement to make sure you are not laundering money.