“They have value in them as well, but we have to look at the best value for the club. We don’t sell anyone cheap.
“At this point it is still quite quiet, but you can feel the interest is coming.”
Earlier in the summer highly rated keeper Marshall and Gabon centre back Ecuele Manga were the subject of transfer speculation.
Asked whether the club were keen to hold on to those players, Choo said: “We will try, but if a big offer comes in we have to look at it from a business point of view, like everything else.”
Cardiff finished eighth in the Championship last season, with Russell Slade removed as manager on the final day.
Choo says the club are targeting promotion this term.
“We are trying to improve from the last two seasons but we are trying to do it progressively,” he added.
“Rather than pumping in £15m we are trying to do it in stages and sensibly. Previously things were not so smooth.
“The last few years we have been trying to make more sensible decisions and Vincent Tan (the owner) is taking his time and studying things a lot more. He is learning, since this is his seventh year owning this club.
“Promotion is the target, there is no point being in the play offs without being promoted. No owner in the Championship wants to stay in the Championship – they want to get promoted from this league.”