Authenticity - How do I know something is genuine?
You don't.... Even the most experienced dealers, museums and collectors can be fooled by photographs or even an actual item, it is not possible to be confident in an item without handling it, and even then there may be concerns which require further investigation. This means that you must trust the person selling the item and be prepared to return an item if you believe it to be fake or unsatisfactory.
If you are not confident in your ability to detect fakes, take the items to a good museum, dealer or auctioneer with a specialist antiquities department. Building a long term dealing relationship with someone you trust is the best way forward. Double-check your purchases with another specialist: good dealers are never offended by their clients' concerns for authenticity. Anyone can write almost anything on the internet, the lure of a bargain can temporarily dazzle so apply a little caution before you part with your hard-earned money.
Provenance: Why does it matter?
Provenance is the term used to describe the history of an archaeological object since it was excavated.
This can include where and when it was found, by whom it was owned, where it may have been exhibited or published. Provenance for antiquities is often modest (i.e. "from a deceased estate in London"), but what seems banal now may be interesting to a collector or researcher in 100 years, so always keep paper-work associated with an item and supply it with the item if you decide to part with it.
Provenance adds value to an antiquity, and the day when un-provenanced antiquities become un-tradable is rapidly approaching. Un-provenanced objects are generally cheaper, but this is because they will always have a lower market value and most reputable dealers and auctioneers will not buy or sell them. Many countries already ban the importation of un-provenanced antiquities: it makes ethical as well as financial sense to ensure you avoid smuggled goods in your collection.