Is it normal for the carrier to not outright buy the phone until they sell it?
I don't know about phones, but in the distribution world, it is very common for a reseller to not actually buy a product before it is sold.
Many companies these days work on a virtual inventory basis with their primary supply chain. The basic idea is that the seller of the product effectively leases space in the warehouse to the supplier with a contract such that the supplier agrees to maintain a certain amount of virtual inventory. When the seller sells-through a product, they don't actually have to pay for the inventory until the second the unit is "pulled" from this hub and then the supplier bills the seller and is on the hook to replenish this inventory. Of course the seller discontinues that product, then it just never pulls any more units from hub and the supplier is left holding the bag (even though the inventory is in the seller's warehouse). On the sale, the seller often still has "net-90" days to pay for it as well. As you can see, the life of the supplier isn't easy, nowdays they need to pay for both the inventory and the account receivables side...
For the inventory on the shelf there is a similar paradigm, as part of the shelf stocking agreement, a repurchase agreement is made that the seller can require the supplier to purchase back some or all of the inventory (although usually at a discounted rate), if the inventory hasn't been sold in a certain number of days. This type of stock/repurchase agreements happens in industries far and wide, supermarkets to bookseller to electronic's retailers.
The rationale for the seller offering a high repurchase price and percentages is for the seller and supplier to maximise the amount of product on the shelves (to prevent out-of-stock sales loss) given the seller's risk tolerance for the product. Of course the supplier may be irrational, but the seller is covered a bit in this case... Usually the seller says I'll risk $X to stock your product on the shelf and the agreement is structured by the supplier that although $Y of inventory is stocked, $Y - #units X repurchase_price = $X.