This gets very complex, do check with your local Customs office, but here are some experiences I've had...
I recently bought my wife a Garden Party in Chicago, and had to take it back through customs in Heathrow. So, declare it and pay duty and VAT, or risk it in the suitcase? Well, as it was in a very large orange box I decided to be honest and pay up. When I got to the red channel I remembered that duty is very complex - should I be charged 3.5% for handbags or 3% for leather goods? Because I had 'fessed up the inspector allowed me to choose the cheaper option. Similarly I persuaded them to only charge VAT on the price before sales tax. Lesson: talk to customs, they may be nice.
On another occassion my wife sold a snakeskin belt on ebay, and shipped it to the US. However it was stopped by customs who decided that it was an endangered species - it wasn't, but customs have complete discretion on this point. They destroyed it (or possibly kept it for themselves, but lets assume they are honest!) Lesson: if your bag looks like it could be made of anything covered by CITES then get some documentation that it isn't.