They are buying the stock, just not in one hit, but as a sell or return. As quoted below.
They receive the £30 per item, then pay the £27. All that is different here is, instead of buying say 100 items in bulk in one hit, the supplier is basically easing the cash flow, and only requiring payment on sold goods.
If the £30 was going directly to the supplier, then commission of £3 per item would only be charged, received by @Scawfell hence then not buying the goods.
Just work to the most easiest for you, and don’t over complicate things.