I travelled from Philippines to UK transit in Istanbul. I bought some diamonds in Philippines properly declared with customs to bring out and will pay VAT in UK.
Q: Do I need to declare in Istanbul while transiting?
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If you remain in the transit area:
You are checked in for both flights
You don’t need to re-check your luggage
you won’t pass customs so you don’t need to declare it in Istanbul.
Rule of thumb, declarations are made to customs, after passport control.
If you're changing airports in Istanbul, ie from IST/LTFM to SAW/LTFJ, you'll be going though customs. Then you will need to declare.
If you're remaining in the same airport transit area, you won't need to declare.