I am thinking of selling my chametz online, and wanted to know the Halachic basis for it, without a kinyan?
I am unaware of the details of the particular transaction that you wish to do.
Nonetheless, in general if one is appointing a Rabbi to sell your chometz on your behalf, you yourself need do no kinyan. The Rabbi does not buy your chometz and then sell it to a gentile, but rather he acts as your agent to sell your chometz to the gentile. The kinyanim that he performs with the gentile are done on your behalf, and you yourself do not actually need to do any other kinyan; all that is required is that you appoint the Rabbis as your agent to sell the chometz on your behalf (apart, of course, from the requirement to define the chometz that you are selling and the place where it is being kept).