Two and a half or three times the scrap value for silver is understandable as the scrap value even though inflated is still low, but anyone paying say three times the scrap value of any karat of second hand gold are paying way over the odds and lots of money to pay it, but if it's a new item and they really like it then why not! I bought that 18 karat bracelet in the above photo about 2 years ago for roughly £1,100 knowing it'd only be worth half of that if I wanted to sell it, but I liked it so I bought it.
Yes, but it depends what the item actually is, not just how much it's worth as scrap. Things go in and out of fashion, and so desirability changes equally. Silver tea services are a case in point. A high Victorian one that's heavily worked with scrolls and flowers will only sell for scrap or little above, as a) the style is deeply unfashionable and b) who uses them these days. Whereas an art deco service, being cleaner and more streamlined in design, will sell for more, based purely on the aesthetic.
If you're buying to use or wear, then yes, I totally get that. Because you can't put a price on enjoyment. If it makes you smile, then why the hell not. And only you can answer as to just how much that smile is actually worth.
As for hallmarks and makers' marks, hell yes if possible. I'm not sure what the threshold for gold is, but silver items under seven grams don't need to be hallmarked according to UK law, but the better makers will do so anyway. On a really good quality and heavy piece, even a chain, every *single* link will be marked.