There is no exchange feature. No mechanism anywhere that swaps one item for another in a single step.
What there is: a return, which refunds you, and a separate purchase. Those two things together are what an exchange is anyway.
Why it is not one button
An auction has one winner. The other item you want was not won by you — somebody else may be bidding on it right now.
Prices differ, so a swap is rarely even. Which means a payment or a refund, which means it is two transactions wearing one name.
Returns are decided on the item you have. Entangling a second item makes a straightforward case complicated.
The two ways to get what you want
1. Return it and buy the other thing. Clean, and your protection on each is intact. The only cost is the gap while the refund comes back.
2. Ask the store. This is the one people skip. A store with the thing you actually wanted will often sort it out — send the right one, take the wrong one back, and square the difference through a private offer.
Message them before opening a return, because afterwards it is harder to combine.
When the store is at fault
If the wrong item was sent, or it is not as described, that is not an exchange question — it is a return you are entitled to, and the store should be covering the postage. Do not let it be reframed as a favour. The wrong item was sent
What to ask for instead of "exchange"
Be concrete, because the word means different things to different people:
- "Can I send this back and take the blue one, paying the difference?"
- "Can I return this and have a credit against your next show?" — some stores will, some will not. Can I get store credit?
- "Can you hold the other one while this is in the post?"
Each of those is answerable. "Can I exchange it?" is not.
Sizing, which is most of this question
For clothing and shoes, the best fix is before the purchase: ask for the measurements rather than the size. A measured chest or insole length is a fact; a size label is an opinion that varies by maker. Can I ask for more photos before bidding?
More: how returns work, can I get store credit, who pays return shipping.