**Edit:** A few people have argued that this question is off-topic, and as I write this it looks like it will be closed soon. However, I have written a new (and hopefully better) version of the question at [How could an inter-temporal banking system work?](https://worldbuilding.stackexchange.com/questions/114178/how-could-an-inter-temporal-banking-system-work), and I hope that you will take a look there.

**Original question below:**

Suppose you have a working time machine. You decide to go back 50 years and buy a house; after all, in 1968 the average cost of a new house was [just about $15,000](http://www.thepeoplehistory.com/1968.html), quite a bargain!

The problem: How do you transfer $15,000 back in time with you? Cash, obviously, won't work, unless you somehow have access to paper currency that was printed before 1968. (Not likely: [the lifespan of paper money is ~10-15 years](https://www.federalreserve.gov/faqs/how-long-is-the-life-span-of-us-paper-money.htm), so good luck finding large quantities of 50-year-old cash.)

Maybe you could bring gold back with you? Not a good idea -- gold currently costs about 30 times more per ounce than it did in 1968, so in order to have bring back enough gold to buy a \$15,000 house, you'd need to start with $450,000 worth of gold.

So -- how would you transport $15,000 worth of wealth back in time with you to 1968?

**Edited:** Many of the answers below are really addressing the broader question "How could you get rich using time travel?" Quite a few of them actually involve transferring wealth from the *past* to the *present*, or from the *further past* to the *more recent past*. The question was intended to be more specific than that; if it helps to focus, suppose you have a single-use time machine, and that you are planning to travel to 1968 and retire there. The cost of living then is relatively low; you just need to figure out how to get your 2018 wealth back to 1968 in a usable form. "Buy something that's cheap now but expensive then" is a good model, but what, specifically, could you bring and sell in sufficient quantities?