A semi-informed reply:
Qmail and sendmail can exist on the same system, and the qmail install instructions walk you through first installing qmail, then gradually migrating from sendmail to qmail.
ecommerce/tcl/qmail-procs.tcl requires qmail-inject. In general, this is not necessary for outgoing mail with qmail, because qmail provides a sendmail lookalike. Other parts of OpenACS, such as acs-mail-lite, use that. Does anyone know why ecommerce uses qmail-inject?
Your options:
1) Install qmail; don't do any of the migration steps. It should work.
2) Modify the qmail-procs.tcl commands to look more like
ns_sendmail $to_addr $from_addr $subject $body $eh $bcc\
and less like
set qmail_pipe [open "| /var/qmail/bin/qmail-inject -h -f$from" "w"]
puts -nonewline $qmail_pipe $msg
close $qmail_pipe