Since you don't want to go through the trouble of sorting the bulk lot, you can sell it as is either locally or at an auction site like eBay. It is perfectly normal to sell bulk lots of LEGO both locally and online without being able to tell exactly what's in the lot. If you want to spend the least amount of time on this, I would suggest first listing the lot on Craigslist or similar venues you may have locally. If it doesn't sell locally within a month or so, put the lot up on eBay.
Whether you sell locally or through an auction site, describe the lot as much as you can, but don't make guarantees. Just describe what you see. If you have a scale, measure the weight of the loose LEGO pieces. Many people buy LEGO by the pound, and knowing that the lot includes i.e. 10 pounds of pieces can be helpful for buyers to understand what they are looking at. (Just for your information, $5/lbs is the usual going price for mixed lot of used LEGO that buyers are willing to pay).
You can sell the lot just by a brief description, the weight, and pictures. It is a normal and accepted practice to sell mixed lots of bulk LEGO. However, if you spend a bit more time on the description, you will likely get better offers. This would include things like the general condition of the pieces, instructions and minifigures. Are there any other toys or foreign objects mixed in? Do the pieces need cleaning? Do they appear lightly used or heavily played with? Are printed pieces and clear pieces scratched up or appear to be in good condition?
If you feel they may have special value, describe the minifigs separately. How many there are, and what is their condition. If you see teeth-marks, paint marks, stains, missing body parts, etc. the value is going to be a lot less then if they are in pristine condition. Again, don't make guarantees, because everyone will have different standards, but just describe what you see.
Include a list of the instruction booklets. List the set numbers printed at the front of the instructions, as well as the condition of the instructions (i.e. missing pages, ripped pages, other damage). The list can give people an idea what pieces might be in the lot.
Whether you advertise locally or through an auction site like eBay, one of the most important part of your description is pictures. The more detailed pictures you take, the better. This doesn't mean having to photograph every piece separately, but just spread out the pieces as reasonably as you can and take good quality shots. You might want to also take separate pictures of the minifigs and the instruction booklets. Make sure that the pictures are not fuzzy and dark. The more people can see, the better their offers are going to be.
If you list on Craigslist or a similar local site, you will need to put a price on the lot. This is a bit tricky as you don't want to ask for something outrageous to put off people from even contacting you, but you also don't want to lowball yourself. You could work with the $5/pound standard for bulk lots, and maybe ask for $7/pound, leaving some room to go lower if necessary. eBay is a lot easier in regards to pricing, as the market will give you exactly what the lot is worth based on your description and pictures. List the lot as an auction with a low starting price and/or reserve, and let people bid what they think the lot is worth. Keep in mind that eBay will take 10% of the proceeds, and another 3% will go to PayPal.
If you sell through eBay, you will also need to ship the lot to the buyer. Get the right size shipping box and packing materials beforehand so you get at least a rough idea of how much it is going to cost to ship. Pack smaller lots into Ziplock bags before placing them in the shipping box. This will prevent the pieces from rattling around too much and getting damaged, and it also prevents bringing attention to the content of the box (stealing boxes full of LEGO is pretty common). You can offer free shipping (this will bring in more potential bidders, but you will have to eat the cost yourself), or let the buyer pay for shipping. The eBay selling platform helps you set this all up.
Keep in mind that selling the lot bulk is still going to bring in the least amount of money. This is understandable, as people are talking the risk of not knowing what's exactly in the lot, the condition of the pieces, minifigs and instructions, and how many of the sets are complete.
Another option you might consider is hire someone locally to sort the pieces for you into sets that you can then sell separately. You might not have the time and inclination, but many people love to sort LEGO. You can even have a sorting party with a few older kids and/or adults. Offer some pizza and drinks and they will be happy to sort the pieces for you into sets. Once the sets are sorted, depending on how complete they are, you can either sell them as complete used sets, partial used sets, or smaller bulk lots of Star Wars sets, Harry Potter sets, etc.
One last thing I would add is that LEGO is bringing back Harry Potter this year. They will likely have much better sets and minifigs than in the past. If you want to sell the lot you have for the best price, I would suggest doing it before the new sets are released.
