There has been a Custom's agreement with the UK effectively since the 1850's - the Goverment Website will have ref to the latest version (tariffs are equalised so no restraint on movement of goods though the Island managed to agree a reduced VAT on certain goods that could not be re-exported to UK (eg fish & chips!, repairs to buildings) - there are no restrictions that I know of that don't apply to UK except for a restricted set of agricultural products - fresh milk + cream is major example as otherwise Manx dairying would be destroyed (eg in UK I pay about 28p a pint in supermarket - on Island a fixed 50p for 1 pint of milk) - under Manx Tax law (tho I think this may have been altered in latest budget) there was a certain class of company that could only do business off Island

The key area of difference would be financial sector - there are many books (and high cost consultancies !) describing the various ways the Island can act as a tax haven (licit and illicit as it appears at times)