Assuming the seller is not using ebay's buyer-hating 'Global Shipping Program', which guarantees both exorbitant postage charges and that any import duties / VAT gets charged at source when you buy the item, then you will only be charged anything at all in this country if HM Customs decide to. Whether they do or not can be very hit and miss, however if the seller declares the full value on the parcel's customs declaration, you will almost certainly be charged import duty & VAT, if the value is high enough, plus a Post Office 'handling fee'. However, if the seller (as many do, even when not asked to) declares a value below the customs limit, then the chances are high that you won't be charged anything extra. However, I have had parcels arrive from the USA with the full value declared (i.e. one well above the threshold for paying duty), and yet not been charged import duty / VAT, and conversely I have received goods where the value was declared as being below the threshold, and yet been charged duty.
I'm sure someone will correct me if I'm wrong, but I believe HM Customs have the right to open your parcel to inspect the contents and to charge pretty much whatever the hell they like, since they often decide on the value themselves (although they can also ask you for proof of its value). Also, the value of the goods upon which they base their decisions INCLUDES the postal charge (i.e. if your item costs $30, but the shipping is $20, then Customs will work on the value being $50, etc.).
Here's what eBay has to say on the subject (
https://www.ebay.co.uk/gds/UK-Buyin...y-Parcel-Force-Fees-/10000000013589137/g.html):
"If you buy goods online from outside the EU for delivery to the UK, you'll have to pay Customs Duty (if over £120 value) and Import VAT (if over £18) on top of the purchase price (including duties), though Customs Duty is waived if the amount of the calculated duty payable is £7 or less. Please note there is also an £8 Parcel Force Fee for collecting the Import VAT."
However, it's also worth noting the vintage toys imported into the UK should apparently actually be charged VAT at a lower rate of 5%, rather than 20%, as helpfully described by pmsbull here:
http://starwarsforum.co.uk/viewtopic.php?f=1&t=36617
Hope that helps and good luck