What is the ruling on recording and selling Amr Khaled's lecture series on CDs at a symbolic price for the purpose of da'wah (inviting to Islam) and spreading knowledge? Is it obligatory to seek his permission for this? And what about the CDs that have already been sold if the matter is impermissible?

If the lecturer permits the copying, selling, and distribution of his lectures, then there is no harm in that. However, if he does not permit it, then it is not permissible to copy and sell them, as it infringes upon the rights of others. In this case, one must seek permission from the rights holder or return his rights to him, because this is one of the conditions for the validity of repentance concerning the rights of others.