Do barter transactions need to be declared for tax purposes, and how is taxable value decided? What is the legality of bartering goods and services in a non-business setting? Say I'm on Craigslist, and I trade my car for a service or another product. Do I need to declare that on my taxes, and if so, how do I assess the market value of those goods or services?
Both of these answers assume U.S. law. Craigslist has no liability per a statute commonly known as Section 230. In a barter transaction, each party must declare as income on their tax return the fair market value of the goods or services received, by a commercially reasonable means (subject to audit), less any basis (or, in the case of accrual accounting taxpayers, at least, any cost of good sold expense deduction) applicable to goods parted with. Under changes to the tax law effective January 1, 2018, this answer applies even if the barter involved tangible personal property of "like kind" which was previously not taxable under Section 1031 of the Internal Revenue Code. Section 1031 continues to provide special treatment to barters involving investment real estate.
If you cannot legally purchase a gun in Utah due to any restriction, such as residency, and you engage someone else to knowingly buy or gift you a gun (such as a "straw man" purchase from a dealer or private sale), that is illegal. From the same link you posted (my emphasis): Can I buy a firearm as a gift for someone? Yes, as long as the receiver is not a prohibited person and the gifting is not being used to circumvent a background check or other laws. Calling a purchase intended in place of another is a straw purchase.
There is a form, which both parties to the sale sign. Note that it does entail paying sales tax. There are other aspects to the sale which reinforce the "It's not my car anymore" message, such as returning the plates and getting a receipt for the plates. Even doing the sale in a DMV lot would not help you.
When the buyer hears &quot;to avoid sales tax&quot; and hears the seller describing as a gift something that s/he knows was nothing of the sort, a reasonable buyer would know that this was at best suspicious, and would find out if this was legal. But assuming that the buyer accepted the seller's suggestion ignorantly and in good faith, the buyer can file a report of the transaction and pay any tax that should have been paid. The exact way of doing this will vary based on the laws of the buyer's jurisdiction (state-level if in the US) which is not given in the question. Such a report and payment might not prevent the local tax authorities from assessing an additional penalty. Criminal charges seem unlikely, but would be possible. If the seller was a used car dealer, the authorities might want the buyer to testify against the seller. They buyer might be wise to consult a lawyer with expertise in sales tax issues in the buyer's jurisdiction. Such a lawyer could advise on exactly what the buyer's legal obligation was, and how to avoid or minimize penalties or other consequences.
Yes, money received from Kickstarter campaigns is usually considered taxable income. Who pays the taxes and how much (if any) they pay depends on how much the teenager makes, how old they are, and whether they are claimed as a dependant by someone else. For information about taxing children see IRS Pub. 929. As pointed out in the comments, certain campaign contributions could be considered nontaxable "gifts" by the IRS. There are also taxable gifts but the taxes are paid and reported by the gift giver. You will want to talk to a tax professional about these. It is worth noting that interest on gifts is taxable unearned income unless the gifts were set up under a UGMA trust, which has its own rules. You can deduct most expenses from income, as pointed out in the comments. This too is covered by Pub. 929.
At the federal level, gift cards seem to be treated as a special case of electronic fund transfer. 15 U.S. Code § 1693l–1 and the corresponding regulations presently regulate disclosure of fees and expiration, and do not directly say anything about sale of such a card. There might be relevant state laws, though Washington state laws mostly mirror the federal law (also allowing issuing a card with an expiration date if given for no value to a charitable organization). You can actually get your last $5 back in cash in Washington per RCW 19.240.020. A gift card / certificate isn't a "thing" in the way that an apple, hammer or table is, it's a contractual relationship. If you own a thing, you can freely re-sell it to whoever you want (assuming there isn't an express statutory prohibition against the sale of the thing). You cannot universally sell (assign) a contract right – there is a default preference that you should be able to, but Amazon has in this case said "No, you can't".
Is this legal? Generally yes, unless it unlawfully exceeds the scope of the license. Also, if it is, how can I check if it's permitted by the original store's EULA? Read the whole EULA, focusing on terms related to resale, assignment, and transferability.
I emailed the Bureau of Alcohol, Tobacco and Firearms in the USA asking this and received the reply: "There is nothing to prevent you from marketing antiques in the UK while present in the US." I also contacted a relevant US attorney and received: "There’s nothing in your question which would implicate Colorado or US law. If you're a UK citizen, and the sale is happening in the UK while you're in the US, then the US has anything to do with it. So as long as the UK doesn’t have any problem with it, the US will not." So I would conclude this question fairly answered.