The "Travelling Cart" is a shop in "Stardew Valley", which
consists of a wagon, pulled by a pig and is occupied by the Traveling Merchant.

The "Travelling Cart" appears south of The Farm, in Cindersap Forest near the
pond.

You can get there by using the bottom entrance to your farm or the path at the bottom left of pelican town.

The "Travelling Cart" is only avalable on Fridays and Sundays between 6:00AM and 8:00PM.

The "Travelling Cart" sells a variety of items, from crops, foraging items, special fish, furniture, and seeds, to Rarecrows.
The items are only available in limited quantities and mostly at inflated prices that vary from visit to visit.
But sometimes you could get items cheaper than normal price.
Additionally I shoud mention that the "Traveling Cart" is the only shop in the entire game that sells Rare Seeds.

For more info about the whole sortiment and the changing prices click this Link[stardewvalleywiki.com]!