You will find that they have a hit list of certain suppliers that they watch out for, so you may well of been caught in that, or of course it could well of been a random find.
Goods from China will usually attract some form attention from customs, at least multiple goods of the same kind usually will because customs know what to look out for.
It's true, some do get away with it, but as time goes by brand owners are really starting to step up on clamping down on fakes, they are simply taking too much of the brand owners market, so customs should only be getting tighter on loads really.
You're pretty safe with anything that is not branded, if it's a replica of a branded product then that's fine, it just can't have any trade mark infringements such as brand name, logo, that type of thing.
Not an easy market to crack, you need to find the right replica, the right price, the right supplier and then hope like hatties that the market is still strong by the time you start selling : )