I have a yam malta, which I think is 3,5hp and I think its about 5/6 yrs old and paid a mate £175 for it, which I rated as a bril. I had been looking for a while and think £300 is Ok.
I could be wrong, but I don't think Yam have made any 3.5HP 2 strokes for quite sometime (about 2003?). Even then £300 doesn't seem a silly price if all is in order. There should be a plate on it with the "model number" etc. This has a magic letter which tells you the year of build... (google "yamaha outboard letter code year" should get you the "key").