You decide that you don't trust the lady.
"How am I supposed to know that you even have this potion of which you speak?" You ask her.
"Isn't my word good enough?" she asks.
"Hey, I don't trust people I don't know, but if you show me the potion, then I'll believe you." You tell her.
She's reluctant and gives you an evil stare but knows that her skill in gypsy magic, other than potions, is lacking.
"Fine, I'll show you where it is."
She leads you back behind the shop and pulls out a bottle.
"This will make you all of those things I promised." She tells you. "But you may find this one just as tempting." She adds, once again turning around.