AuctionCheckoutComplete and FixedPriceTransaction are the two events that you can evaluate based on your exact requirements.
Both are triggered when buyer purchases and item and both are applicable for FixedPriceItem. AuctionCheckoutComplete is applicable for auction items too.
However, there are some pros and cons for both : For example: please refer to https://ebaydts.com/eBayKBDetails?KBid=1041 for AuctionCheckoutComplete notification to see the limitation.
I believe you have already requested this information in your support ticket, so you will get more details in the ticket.Thanks Vijayta
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