I have several complete Lego sets that are assembled. I would like to sell them and want to know if I should ask a higher price if I disassemble them and organize the pieces? I also have the instruction books that go with them.
A good part of the LEGO experience is building the set. There is little joy in buying a LEGO set that was already built by someone else. So you are right, that a disassembled set is usually viewed as more valuable. However, consider the following points.
When people purchase a LEGO set, either new or used, they expect the parts to be sorted in some way into bags. So, taking apart the sets you have and putting them into baggies is not considered an additional service you're providing and buyers are willing to pay extra for.
Shipping a fully assembled set puts a lot more liability on you than shipping loose parts. If you advertise the set as fully built, and someone buys it, if any of the set gets broken or pieces fall off (which will likely happen in shipping), the buyer has all the rights to be disappointed and demand a resolution from you (shipping the set back to you at your expense, getting a partial or full refund, etc.) Also consider that shipping a fully built set is going to be more expensive, as you will have to use a lot more protective wrapping and a larger box.
As far as how much you can sell the sets for, that will be dictated by the market. You can certainly start out by listing your sets at the higher end compared to other listings, but it will be up to the buyers to decide if its worth it to them. If they don't buy, you will have to consider lowering the price, keeping the set for yourself, or selling the parts in bulk or individually.
If you are looking for angles to make your listing more attractive, here are some things buyers are looking for and willing to pay more for: Clean pieces, no damage or fading to the pieces, minifigs, instructions, and box (if provided). Stickers are clean and nicely applied without any fading or curling. The pieces are nicely organized in baggies based on color, piece type, or building steps. The set comes from a smoke and pet free home. The set belonged to an adult collector (or if from a child, the pieces are cleaned and sanitized). There are clear and detailed pictures included with the listing.
Hope this helps some. Any other questions, just post it as a follow up, and the community can give you more info.
Let me add another perspective to TheBrickBlogger’s answer: when I buy a used set on EBay, I like to have some sort of a visual guarantee that the set is complete and as described. The only way to get that confirmation is by seeing pictures of the built set, ideally from all angles.
So, when I sell used sets on EBay, I will make sure the set is fully built, take pictures from all angles, take pictures of areas where pieces are missing or stickers are less than perfectly applied, and then list. Then, depending on the size of the set, I take the set apart (potentially sort it, but that is not a must) and make a clear note in my listing that the set is complete (with either “missing the extra pieces” or “missing minifigs” or something like that if necessary) and will be shipped disassembled in a ziploc bag.
By including pictures of the built set, I give the buyer a visual confirmation of what they can receive, and at the same time insure myself against “but the minifigs are missing” claims that can be easily disproven by just looking at the picture.
As a buyer, even if the set is built when I receive it, I will still disassemble the set, wash the pieces, and do a full inventory to make sure all pieces are included (using the inventory at the end of the manual, or using the BrickLink inventory in BrickStock). So there is no difference for me between a set that is shipped assembled or disassembled, apart perhaps for the extra effort in taking a built set apart.