Re: mercury 40 hp
1 - from the decals style it's from 2001 and up (you could exactly tell the year by the serial number...)
2 - it's a 2-strokes ELPTO model (Electric start, Long shaft, Power trim, Oil injected, we can see the starter, the power trim swith and the oil tank on your third pic), so yes it has power trim and the tank is for the oil that mix with the fuel.
3 - from the appearance of the nuts on the powerhead, it look like it has never been ?opened?.
4 - there's a lot of white deposits -> salt water use
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5- as for the asked price, really, i don't know. It's all relative to your specific market supply and demand. Personnaly, i'd stay away from a saltwater engine of unknown history...