Well, I believe it doesn't have to be pointed at any URL that is specific to the reseller. The same url that you access whm should work:
http://mainip/whm
and just enter in the reseller's u/p.
BTW, if you are transferring domains, you should look into using the Transfer tools in WHM. I've only used it a couple times against another CPanel server (not my own), but have had success with it. Makes things much easier.