Following in the vein of the recent time travel threads, what modern items could you use in trade with the past?

If you could establish a “Trading Portal” with an era, what would you use to trade if you are restricted to nothing in advance of the technology of that era? For instance, gold was very inexpensive in the fifties and sixties in relation to current pricing. Seventeenth century furniture, artworks, weapons would be worth an incredible amount of money nowadays if in as new condition. What could you take from here to trade? If you were clever you might analyse the market and make a fortune – even just before the Great Depression, as long as you had seed capital. Drugs – something as simple as Aspirin might be one possibility.