No, <p>No, sadly if you sell a item for NPC you couldnt undo the action.
I action.<br><br>I can tell by my own experience when I sold by mistake my Horn <img alt="" src="https://vignette.wikia.nocookie.net/tibia/images/8/8f/Horn_%28Ring%29.gif/revision/latest?cb=20130903162130&path-prefix=en" style="height:32px; width:32px">Horn to Rashid <img alt="" src="https://vignette.wikia.nocookie.net/tibia/images/f/f5/Rashid.gif/revision/latest?cb=20090620190414&path-prefix=en" style="height:50px; width:41px">Rashid for 300 gp.gp.</p>