Is it illegal to do this on eBay?

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You get a friend or family member to bid on your items, thus driving the price up. The item will then be sold for a higher price than it would have done without the help of your friend.

If the plan fails and they end up winning the bid, they can pay to make it look legit, and then you just give them their money back in person. Nobody loses out and you just relist the item and do it over again.

Is this against ebay's terms?

If so, how would eBay even find out? I have thought about it and it seems there is literally no way they could know or prevent this.