Relo properties are rarely prepared for market and sell "as is". So a buyer is going into a vacant house and every bit of scuffed paint, carpet wear, and dirty cabinet is there to see.
At one time, I did a study of relo sales and could prove that, at that time and in that market, relo properties sold, on average, 6% below non-relo properties.
Just like any other sale, you need to do the analysis before you can call it a comp.
As an aside, the definition of market value applies to the subject property. Sale prices for potential comparables may need to be adjusted for financing or buyer and seller motivations. Just because a buyer or seller of a potential comparable was unduly motivated, the sale should not be automatically rejected. Too many appraisers do not consider adjustments for condition or seller motivation. This is why there is such an uproar about the use of REO sales. The problem isn't in their use. The problem is not making appropriate adjustments for factors that affect their sale price when performing a market value appraisal and using REOs as comparables.