contact your Seller..tell him your strings were rusty (and see if he cares enough about Feedback to replace them)

In Feedback (presuming the Seller doesn't want to replace..but some Sellers DO make a little bit of effort sometimes--nothing lost by trying) leave a message that the strings arrived in a timely manner, packaged well, and rusty. (or leave out the timely/packaging if it's not accurate)

No need to be nasty..but no need to let somebody else end up with a rusty set too.