An Antique is 100 years old - this is the definition used by the UK Customs.
Antiques are only liable for 5% VAT and NO duty. Other items may be charged 20% VAT plus duty of anything from 5% to 12% - remember this is on the value plus the postage. I understand "Collectables" are also zero rated for duty and lower VAT.
Customs and Excise published an updated document in January 2008 which includes further information about items qualifying for the 5% rate - note the Tariff Codes that are helpful if you can get your seller to quote them;HMRC Reference:Notice 702 [excerpt]
* Objects other than works of art or collectors’ items, which are more than 100 years old (tariff code 9706 00 00).
11.3 Collectors’ items
* Postage or revenue stamps, postmarks, first-day covers, pre-stamped stationery and the like, franked or if unfranked not being of legal tender and not being intended for use as legal tender (tariff code 9704 00 00).
* Collections and collectors’ pieces of zoological, botanical, mineralogical, anatomical, historical, archaeological, palaeontological, ethnographic or numismatic interest (tariff code 9705 00 00).
11.4 Items of historical significance
History - the study of past events, especially human affairs.
An article which is not 100 years old may be eligible for the scheme under this heading if it is of historical significance because of it’s uniqueness, or by having a direct association with an historical person or event, or is a rare example marking an important change in technical or artistic development in a particular field. Items, which were mass-produced or are merely the products of a bygone age, are unlikely to be eligible.
So, if at all possible, you need your sender to declare the item as "Antique Doll" (or parts, head etc.) and preferably give a date such as c.1900, 1900 -1905, 1890s. If your sender is unwilling to write "Antique" then ask them to write "Collectors Item" instead. Remember: OLD, USED and VINTAGE have absolutely no meaning to Customs - it has to be ANTIQUE or COLLECTORS ITEM.
Ask your seller to declare the value as the winning bid - certainly no higher. It is possible to reclaim overpaid charges, but it takes a lot of time and work. I once had a seller who left out the decimal point when declaring the dollars - you can imagine the payment I had to make and then claim back.
With regard "Gift" or "Merchandise"; strictly speaking all eBay wins are merchandise, although some may be lucky to get a gift. Duty and Tax are payable on GIFT items worth more than £36 and MERCHANDISE worth more than £18. So if your items are worth less than that, they are exempt anyway.
I hope this has helped give you all the information you need, I will get round to writing a guide to claiming back overpaid customs charges soon.