"Can see"? it is impossible. if your reseller is not lamerz he can use some php or perl or just shell command and list all ip's which setup on servers. he also can use remote command for doing same. but, if you talk about "can use any ip" then BPrintz correct - you can restrict reseller to only assigned to him ip adress.
Rustelekom LLC Dedicated server since 2002, RIPE NCC member, LIR, AS51168