More sample code here:
https://openacs.org/forums/message-view?message_id=305818
Also, I looked at IPN enough to get working code, below:
set form [ns_conn form]
for {set i 0} {$i < [ns_set size $form]} {incr i} {
set key [ns_set key $form $i]
set value [ns_set value $form $i]
ns_log Notice "ipn: $key=$value"
}
set qsset [ns_set copy [ns_conn form]]
ns_set put $qsset cmd "_notify-validate"
set results [ns_httppost http://www.sandbox.paypal.com/cgi-bin/webscr "" $qsset]
ns_set free $qsset
ns_log Notice "ipn: results=$results"
ns_return 200 text/plain done
