Aaah, finally something in my area of expertise!
The customs people here are well and truly switched on, I had circa 70 deliveries last year from Ebay purchases (mainly kicks/sneakers and vinyl art so EXACTLY the collectables you describe and I also thought they would never ever know the true value) and on circa 20% of these deliveries my items were held, I recieved a little white card which requested a "receipt or invoice" to prove that the price the seller had quoted was accurate.
I called and spoke to someone there and they have a PC now where they will search for your purchase on Ebay (often since the seller includes the Paypal receipt inside the package, if not they'll use the item number) and if they don't match then expect a stern telling off and all of your future purchases to come under this type of scrutiny.