Selling complete sets, Assembled or Disassembled?
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.