Usually i bought bitcoin when the rate value is low, and sell it when the market value is high. I bought bitcoin for the purpose of trading ,investment and gambling. But when the value of bitcoin is in high price i didn't buy, i prefer to go on faucet for saving bitcoin.
Good idea that buy bitcoin with low price then sell it with high price mean its trading is very popular that people buy bitcoin with low price and then wait for until increase its price to higher and i think its good way to earn profit from bitcoin.
The problem is no one can judge what is the all time low price to buy bitcoins and all time high to sell them, if you feel that price is low for you and you can afford to invest in it then you can go for it.