There are two issues here.
Regarding the first house you either paid well over the market price for such a property or something has happened in the area which means prices have fallen even though prices elsewhere have risen. A mortgage type survey /valuation report would have helped.
Regarding the second house, yes you do need 2 or 3 valuations. Estate agents try to make you feel your house is wonderful and will try to make you feel even better by starting with a high price. Some just go for volume- get lots of properties on the books and by the law of averages some will sell and many won't unless the price is reduced. Overvaluing by 10-20% is common and then you have the problem that some potential buyers can suck you in and then knock you down again at the last moment using their survey findings as justification.