I think there is too much stuff (not a criticism; we have about ten times more stuff than you do crammed in, but then we're not trying to sell...)
Based on far too many hours frittered away wasting the House Doctor, I'd say
Photo 3 -- there is a lot of stuff. There's the unit, and two sets of bookshelves, and the DVD unit under the television, and the white storage chest. It makes the room look smaller and it raises a red flag that the house doesn't have enough storage space. If I were being ruthless I'd put the unit and the storage chest into a storage facility somewhere, thin down the contents of the shelves so that there are some books/DVDs but also some knick-knacks and some space, empty out the DVD shelving under the TV (some to go onto edited main shelves, rest to go into storage along with many of the books from the shelves). And you need to sort out the fireplace area somehow (and I'd try to beg, steal or borrow a big mirror to go over the fireplace, because of too much Ann Maurice).
Photo 4 oooh, and there's another storage unit of some kind lurking behind the armchair, and it looks as though it's got more books and stuff on it. Thin that out as well you want to be sending out "all our stuff is invisibly stowed away because this house has SO MUCH SPACE" vibes.
Photo 5 (and 7) -- more storage units. If you can, put them in storage temporarily. And think about staging the dining table for dining (plates, glasses, maybe something in the middle of the table).
Photo 6 (and 8) -- kitchen isn't huge, but it's not cluttered. Don't think you need to do anything there.
Photo 9/10/11 -- it's a family garden, probably what most viewers are expecting.
Photo 12 (and 15) -- no problems here. It's not an enormous room but it's not cluttered.
Photo 14 -- it's a small bathroom, but not much you can do about that.
Photo 16 -- Nice; good size for a child's room.
Photo 17 -- I see what others are saying; it doesn't look as though the room's big enough to be used as an actual bedroom. I'd be tempted to move the cot into Bedroom 2 alongside other child, borrow or hire a single bed from somewhere and style this room as a single bedroom to prove that it can function as one.