The process for negotiating the price of used rims is the same as any other type of price negotiation: express your interest in the item, make an offer and don't be afraid to walk away from the deal. Inspect the rims before purchase to make sure they're not severely warped. Used rims typically have at least moderate warping, but you can use this to negotiate a lower price.
If you're responding to an advertisement, start by contacting the seller via email or phone. Email is preferable, as it allows you to take your time before responding. If you have any questions about the rims, ask those in the first email to verify that the rims meet your needs. Otherwise, make an offer for the rims in that email. If you ask questions in the first email, wait a couple days after the seller responds to email back with an offer.
Offer an amount that you're willing to pay that's less than the asking price. If it's significantly less, explain that you realize it's not what the seller is asking, but you felt you should offer just in case.
If the seller accepts your offer, the deal is done. If the seller counters your offer, choose whether you want to accept it, raise your offer or decline. When you decline, tell the seller that's out of your budget and to let you know if he changes his mind. Sellers often get desperate if they can't sell something, which leads to them accepting lower offers.