There is no universal definition for which manufactured items are antiques. Some tax agencies define goods older than 100 years as antiques.
The definition has geographic variations, where the age limit might be shorter in places such as North America than in Europe. Handicraft products and limited product series might be defined as antiques, though they are younger than similar mass-produced goods.
In some jurisdictions, antique motorcars may legally qualify as historic vehicles after as little as twenty-five years and a few buildings may be eligible for a spot on a historic register after fifty years – but only if there are other substantial, documented reasons for their historic notability. This broader definition includes some ephemera and nostalgia pieces from the twentieth century, including the Great Depression and the post-war 1950s. An automobile from the Roaring Twenties, if it is still in operable, showroom condition today, is indeed an antique.
High-value objects are normally sold in auctions; the major international auction houses are in New York City, London and Hong Kong. Be prepared to pay a premium price since you will be bidding against museums and collectors. National auctions can have good assortments of domestic arts and antiques, although the most famous pieces are sold at the major houses.
Antique shops have mainly items of moderate value. Simple antiques can occasionally be found at flea markets and garage sales.
There are legal restrictions on various sorts of goods which may be sold as antiques:
Knowing the local rules is important. In some cases exceptions may be allowed with appropriate permits, but in general violating these rules can lead to loss of the goods and other consequences – being prohibited from visiting the country again, a fine, or in some cases even a prison term.
Antique stores and auctions usually add a commission on top of the announced price; which can be up to 25 per cent.
Art and antiques usually follow tax laws different from consumer goods. For art and antiques of significant value (where one could expect to sell the property for more than the purchase price) the buyer should get a receipt for future tax reports.
Transporting large or fragile antiques can be more challenging then normal souvenirs. Ensure your items are not exposed to extreme temperatures, harsh light, or rough shocks while transporting them back.
The terms authentic or genuine (in contrast of replica, forged, etc) are very contextual. When it comes to jewelry, it implies they are made by noble metals and gemstones, instead of cheaper materials. For furniture, it implies that they are crafted before industrial times; in contrast to mass-produced goods in similar style. For craft items such as carpets, it implies that they are handmade, without regards to age.
For new items, from the late 19th century onward, there may still be a copyright, in some cases also for older items, especially those that were "published" only recently (check the legal definition for your jurisdiction; not all art was ever published). Usually you buy the item, not the copyright, which means you still need the artist's (or their heirs') permission to make copies, such as publishing photos of it on the internet. Copyright rules vary by country (Wikimedia Commons has some info on most countries), with copyright expiring 70 years after the death of the author ("pma") in most countries, 95 years after publication in the USA, some countries have a 100 years pma term, EU gives 25 years for previously unpublished works, and there are some additional rules in many countries.