First, a look at what the trade goods should be ...
- Needs to be recognized as valuable by the locals.
The suggestion to bring medicines does not quite work since it would either require them to take the properties on faith, or the strange traveler would have to be allowed to practice medicine on people who can pay. Having miracle-cured a couple of sick peasants won't be enough. And what if one of the high-profile patients has an allergic shock?
- Needs to be recognized as non-demonic.
That might be a real problem in some historical medieval times, perhaps less so in a fantasy world with accepted and not-always-black magic. On a related note, if the gadget malfunctions easily, the seller may be blamed or called in to fix it.
- Needs to come in small or divisible units.
A time traveler selling a single gadget to the highest bidder does not sound very medieval to me. He will be in one kingdom, possibly at the court of one king or queen, and auctioning it between representatives of several kings sounds just too modern-times. Perhaps an auction could be done with the merchant guild in one trading town, if there is such a thing, but that brings middle-men into the profits. Anyway, payment could come in the form of a promise to deliver 1,000 bales of wool next year, not in cold hard cash. Nobody had a king's ransom lying around.
- Needs not to challenge the local balance of power.
A set of radios, if they get accepted by the local warlords as what they are, might put the time traveler in the center of a war of conquest. In the headquarters of the winning army at the beginning, but such things have a tendency to change.
After all those comments on what doesn't work, here is my suggestion. Trade depends on comparative advantage. So what can we make cheaply that the medieval folks would have trouble making?
Small steel gadgets
Sewing needles, safety pins, small scissors, perhaps small nuts and bolts (the threaded bolts would be invaluable to many artisans, and bringing many small ones means they can learn what to do with them).
Colorful inks and dyes
There will be scribes. Small pots of modern inks would allow them to do much more impressive documents. Alternatively colored pencils. Similar for dyers, if they can be taught how to apply the dye to wools and linens.