The halachic transfer of property between parties (chaloth kinyan) conventionally involves a physical exchange of a tangible object (maaseh kinyan). For example, the rabbi of the synagogue I used to attend prior to the apocalypse, when acting as an agent for selling chametz, used to always have the seller sign a tangible contract authorizing his agency. In addition, the agency itself was apparently effected by a kinyan chalipin involving the seller acquiring the pen from the rabbi. Considering the current crisis, is there a universally accepted way for all parties involved to effect a halachic sale remotely, avoiding any need for direct physical contact between the parties?
As Epicentre says, there are multiple opportunities to sell Chometz online without the kinyan chalipin.
See here from the Federation of Synagogues in the UK.
Here you have the version from din-on-line.
Chabad have a form too and say:
Note: Leading halachic authorities have ruled that online authorizations are acceptable for the purpose of selling chametz. The traditional and preferred practice, however, is for the authorization transaction to be done in person with the local rabbi.