How big is it and did you find out what the stamps said.
if is small ,perhaps enough for one or two individual servings , the it is a chines teapot.
Yixing teapots were never glaxed on the inside, the clay would be of Zisha , which is actually a purple hazed clay. The pots were left unglaved or uncoated so that with each brewing , the pot would absorb the flavors into the clay, which is why only one type often would be dedicated to each pot.
Each potter for a Yixing pot would stamp it with his one individual mark...A Chop Mark.
You dont say whether your teapot is old ,as yixing pots are still produced today. Even so, what could be had at fleamarket prices are rapidly increasing in value....Now if its old and lets say from the Guang Period,,,you could be looking at 1500.00 or higher in value