@IWRATS, indeed, Bulgaria has been known in the past for various creative "import/export" schemes.
From your document:
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Free ports are warehouses in free zones, which were – originally – intended as spaces to store merchandise in transit. They have since become popular for the storage of substitute assets, including art, precious stones, antique, gold and wine collections – often on a permanent basis. Apart from secure storage, sales arguments in the free port business include the deferral of import duties and indirect taxes such as VAT or user tax as well as a high degree of secrecy.
The best-known free ports catering for 'investment art' are those in Geneva, Luxembourg, Singapore, Beijing, Monaco and Delaware. It is commonly understood that these jurisdictions consider a free port an addition to their attraction as an offshore financial centre.
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This fits perfectly with London's role as an alleged money washing financial services hub. No wonder the government is keen on free ports...
In related news, Joe Biden was Senator for Delaware from 1973 to 2009. So his attitudes to tax avoidance schemes may be quite "relaxed".