There is a search box in the top right hand corner of the website. Just type in the name of the firearm. You also could add "FS:" people use that quite often in the title for "For Sale"
I'd you are looking for a firearm for sale thèn go into the firearms for sale section. At the top of the individual threads there is a black bar. On the far right side of the bar ther are the words. Search this forum. That will let you search only items for sale and keeps threads popping up where the item was discussed for other reasons.