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i would not sell something to someone for them to make profit from me
Every time you make any exchange, the other party profits from you, regardless of whether they make further exchanges with third parties using the same goods. Fortunately, you also profit from them.
You can go through life ascribing bad motives to market actors and make yourself miserable, or you can stop imagining that you have some mystical insight into their motives and judge them by the results of their actions. A got the item out of your way and paid you more than you'd have settled for, and then gave it to B for less than B thought he might have to pay. A might be a profiteering arsehole, but he has done both you and B favours by accident in the process.
If you sell to A for £10 and A sells to B for £20, then the item has ended up at its best home because B obviously wanted it more than A did. A has done you a favour by finding a better home for your unwanted item than you managed by yourself. You're happy because you wanted £10 more than you wanted the item, A is happy because he also wanted £10 more than he wanted the item, B is happy because he wanted the item more than he wanted £20.