some good advice here so far; things to check track conditon this can set you back 1000-1500 bucks if needed, slide rails and bogie wheels these are wear items and while not expensive need to be checked and replace occasionally.
Make sure you can start the sled you want to look at if not a compression test should be a mandatory part of inspecting a potential sled for purchasing, 2 stroke sled requires a min of 90psi to run should be 110-120 psi, 4 stroke around 160-180 na and 140ish for turbo engines.
check condition of recoil (starter rope) for fraying wear and easy recoil, easy to miss check the condtion of the front skis and runners this can easily set you back 500 bones,
all suspension parts in the front for wear(play) and cracks pay close attention to control arms and mounting points.
Check condition of battery and e starter if equipped (average battery life is 32 months) all lights and thumb and handwarmers for proper function
See if the person has any documentation on the sled for maintenance.
And as for licencing, you will require a annual sticker for your sled, insurance as well if used on public property (lakes) and if you are using ofsc trails you will require a trail permit.
with atv use no permit is required for trail use in the winter on PUBLIC trails as of yet, but you are not permitted on ofsc private trails with an atv (or a sled without a permit) while the trails are open for sleds if i can think of anything else i post later