Where can I buy Coins & Collectibles?

Excuse me, I was wondering if you could point me in the right direction. I'm looking to purchase some coins and collectibles, but I'm not sure where to start. Are there any reputable online marketplaces or physical stores that specialize in these sorts of items? Additionally, could you offer any advice on what to look for when buying coins and collectibles to ensure their authenticity and value? I'm a bit of a novice in this area, so any guidance would be greatly appreciated.