Say you work in a company that buys and sells mobile phones. Customers bring you used mobile phones so that you buy them, repair them if needed, then resell them with a margin. The act of selling is a "sale", as far as I can tell, no problem there. But what about the act of buying ? Would you just call it a "purchase", or is there a more specific word when you buy a used product ?
I would call this "flipping":
- To buy and resell (a house, for example) in a short period of time for a profit.
It's commonly, but not exclusively, used when talking about people buying and re-selling houses. You could certainly say, "She's been making money flipping smartphones on eBay."
These programs are usually for customers of a company that sells both used and new products of the same types. So, a customer who wants a new phone can bring their old one and trade it for some credit toward the new model.
Sometimes the customer can get cash they could spend anywhere, like when I trade in books at Half Price Books. The amount of cash is usually less than the amount of credit you could get toward other products that the store sells. This is meant to encourage you to spend the money at the same store you traded in your used item.
I realize this isn't exactly what you're asking for, because "trading in" is something the seller of the item does, not the buyer. The buyer would accept a trade-in. If the company just buys used items for cash, the actual transaction doesn't have a special word; it's just buying.
There may be special words for how the company finds the used items to buy though. For example, there is a reality TV show called American Pickers that Wikipedia describes as a show that "follows antique and collectible pickers Mike Wolfe and Frank Fritz, who travel around the United States to buy or "pick" various items for resale, for clients, or for their own personal collections."