If you are doing F/F work, you sign a statement that ". I verified, from a disinterested source, all information in this report that was provided by parties who have a financial interest in the sale or financing of the subject property." The little field on the MLS sheet, whch BTW is going away, I don't think is sufficient. Also sometimes the concessins are not $$, they can be weird.
Hre is HUD's take on it. "The appraiser must verify comparable sales. MLS by itself is not considered a verification source. Contacting someone with first-hand knowledge of the transaction (listing or selling agent, buyer, seller, etc.) is the preferred method of verification." Speaking to HUD, I was told if after attempts were made, you can just document your attempts and use your best judgement. But you have to try.