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Comprando.
Are you purchasing?¿Usted está comprando?Are you returning?¿Está regresando?
Te estoy comprando zapatillas de tenis
(ella) compra/esta comprando un sombrero
'ella' is redundant if it's obvious who you mean
'esta comprando' describes the buying at that moment
"I'm buying" in Spanish is "Estoy comprando". It is pronounced "eh-STOY coam-PRAHN-doe". Please see the Related link below for confirmation of the translation.