A dominant undertaking would be obliged to offer the same terms to equivalent outlets, so if two wholesalers both commit to buying the same quantity of spares in the same time period, they would have to be supplied at the same price. Obviously those buying more can expect to get some additional discounts, but the same deal must be offered to anyone who makes the same commitments.
Increased competition does indeed usually lead to lower prices. The wholesale business is all about holding an extensive range of products, and being able to deliver them efficiently to the customer. It is always a balance between tying up money keeping the shelves fully stocked, versus the risk of losing sales if you run out, all of which has to be supported by a very efficient order, handling and fulfilment service. Wholesalers will need to raise their game.