Typically, it's not legally permissible to restrict the price your wholesale customers sell your products for. However, in most countries, you can legally implement an agreement that restricts the minimum price at which they can advertise your products. By establishing a minimum advertised price, you can prevent excessive competition among your wholesale customers and maintain a sustainable market for re-orders.
Take a look at the promotions at retail locations near you. You'll often find marketing campaigns making statements such as "Too hot to advertise, call for pricing!" This indicates that there are likely contractual limitations on advertising the product at excessively low prices.
One of the most prominent companies that employ this strategy is Apple. You'll never see an Apple product advertised at a steep discount compared to other retailers.