They need to fatten him up before the move. I would want to watch him eat and observe his behavior for a bit. I would then ask them to feed him a combo of daily algae strips and frozen brine (algae infused if they have it) for at least a week before the transaction...
And then only if you have a large well established tank, if he is eating well, if its a good price and probably also add another if..if you have a qt tank or if the other occupants of his tank are disease free and it was a well established tank.