Buying low and selling high only works when you think the player is only in a slump and will return to his prior form. I admit he is probably in a slump right now, BUT, he will never return to his MVP type seasons. In this case it's not a case to buy low and sell high, it's a case to cut your losses before they get worse.
However the good thing is that, I bet you could trade him and give him to another owner who has the same logic you do (Giles is slumping, therefore I'm buying him low). If I owned him, I would deal him right now, before his slump continues, the longer you wait, the longer his value suffers. Or, if he has a good game or two, deal him then because everyone will be saying "Giles is back!", when he will never be the same.
Obviously, it's a personal decision regarding how much faith you have in Giles to rebound. I am not saying he won't be a good OF this year, but he will not even come close to justifying where everyone was drafting him, much less have the MVP type season that so many "experts" were predicting. When you play half of your games in Petco, subtract 10 HRs, then factor in his age and subtract another 5.