Generally, the change in finish won't add any value to it for resale, in fact it detracts from it because most people are usually looking for M41's with their original finish.
But in practical terms, it's now a pretty good "working shooter". Depending on what the aftermarket finish could actually be, as there's a variety of ones that can make a gun look like a stainless gun (electroless nickel, brushed hard chrome, NP3, etc), it's now wearing something that's potentially more durable than the original bluing.
$599 puts it in the range of a Browning Buckmark, but the M41 is a better gun overall.
So, if you're looking for an M41 that you can shoot and have fun with, and not worrying about lowering it's resale value as an original gun... I'd consider buying it.
Is the refinish hiding something? Probably not, it could be a fix for an original finish that was rusted from some neglected in some way or it could of been a deliberate decision by a previous owner looking for a finish that was easier to maintain than bluing due to a humid climate. Or perhaps they just wanted a M41 that looked unique.
