It's a good thing for the decals (gold) to be intact, but they aren't always, Where the right hand lays on the machine usually wears away the gold. Also, the faceplate on the end of the machine may be either striated or scrolled. The scroll plate is the most collectible. They were made prior to the war. After the war began, they required less metal for the straight striated plate.
Age is not really a price factor unless its a centennial edition machine. You can determine this by looking at the little decal on the front right of the machine. The centennial medallion is circled in blue vs. black on the other models. Centennial models demand a greater price because of the relative rarity.
The plug where the cord plugs into the machine should be inspected for damage. These are easily broken when the plug is removed. The cord should always be removed straight away from the machine to prevent cracking/breaking the plug. A replacement plug can cost 25-35 dollars.
Look at the black carrying case. Inspect it for an intact handle. The older ones were leather, new or reproduction ones are frequently plastic. Smell the box. If it smells moldy, don't buy it. There may be a faint smell of old glue. (Glue was made from horses hooves.)
Accessories include a variety of pressor feet, a few small tools, (i.e. screwdrivers, foot screws, etc.) It's always good to have an manual with the machine. Many people take them out and sell them independently from the machines.
Also check the bobbin and bobbin case. The bobbin case (where the bobbin fits into the machine) must be there and be intact. These can become bent and therefore useless. The bobbin itself is subject to being bent if dropped. If this happens, it's also trash. Replacing a bobbin case is expensive. Most sellers remove the bobbin while displaying the machines because they are frequently stolen. Alos, check for a key. They are hard to find, but they are out there. The little black case originally had a key for the two locks on them.
I hope this info helps you in your search. Please feel free to contact me if you have any further questions.