Normal people try to earn some quick money from novice buyers on eBay. Happens with a lot off things - they snipe some product(s) that get sold out really quick and later resell those items for a premium.
If you buy from Ebay, you do not need to worry about custom and duty.
I have no problem with customs and duty in the US. My problems with Ebay are all the fraud and scammers - especially on these pre-orders which Ebay doesn't really support.
I bought several Z9 minis on Ebay because I was late to the game. The honest sellers trade information like order number from Bitmain and tracking. One of my orders 45 days ago sent me a USPS priority mail envelope with a code in it. Claims it was to prevent fraud. Then they posted that tracking number on Ebay as product shipped and went silent. It took me several weeks of fighting with Ebay, then PayPal, and finally my Visa card to get my money back.
My advice it to stay away from Ebay for "advance orders". You might run out of time for a claim.