More resellers sounds like more sales. On Amazon, it often works the other way.
On Amazon, every seller shares one product page
On a high street, each shop has its own shelf. Amazon is different: every seller of the same product appears on one product page. Customers see one listing, one set of reviews and one "Add to Basket" button. Behind that button, Amazon decides which seller gets each sale, mainly on price, delivery and seller performance. That spot is known as the Buy Box.
So when you add resellers, you don't add shelf space. You add competitors for the same sale.
What goes wrong
Price wars. Resellers compete for the Buy Box mainly on price. One undercuts, the next follows, and margins shrink until someone stops stocking you, or your other retailers start asking why you're cheaper on Amazon.
Nobody owns the listing. With many sellers on one page, it's unclear who is responsible for the title, images and product details. Listings drift out of date or pick up errors that cost sales.
Poor service lands on your brand. If one reseller ships late or sends damaged stock, the customer doesn't blame the reseller. They leave a one-star review on your product.
Stock gaps. When nobody plans stock, the page swings between too many sellers and none, and products that keep running out tend to slip down Amazon's search results.
Nobody invests. Advertising and listing improvements help every seller on the page, so no single reseller has a reason to pay for them.
What brands can do about it
Register your brand with Amazon. Amazon Brand Registry gives you more control over your listing content, and tools to report counterfeit sellers.
Choose fewer, better partners. You decide who you supply and on what terms. One or two committed partners usually do more for a brand than ten who only compete on price.
Agree expectations in writing. Listing standards, stock levels, customer service, and who handles what.
Recommend prices; don't dictate them. In the UK, you can suggest a retail price, but you can't force resellers to sell at or above it. Fixing resale prices (known as resale price maintenance) is illegal under competition law, and the Competition and Markets Authority has fined suppliers for it. Take advice before you set any pricing policy.
What to ask any reseller who wants to stock you
Will you improve my listing, or just sell on it?
How do you plan stock so the page doesn't run out?
Who handles customer questions and returns?
What happens to my brand on Amazon if our relationship ends?
A good partner will have clear answers, and will care as much about your brand's long-term health as about this month's sales.
Thinking about how your brand is sold on Amazon? Tell us about your brand.