the basic syntax in your case would be
tcpdump -i <interface to capture on> <filters>
The <filters> would expand to something like
'(host 192.168.1.2 or host 192.168.1.3 or host 192.168.1.4) and (port 80 or port 443)'
if your eCommerce application would use ports 80 and 443 for communications. The single quotes are important, otherwise your shell might see the brackets () which are important for grouping parameters as special characters.
adding -v and -n parameters at the beginning (tcpdump -v -n -i ...)would add verbosity to the output and disable name resolution (speeds up output)