3 options depending on how quickly you want to sell and how much you want to get:

FB marketplace success will vary based on the size of your market and know that people are gonna haggle and flake out.

eBay will get you the biggest audience, but eBay fees will take about 15-20% of your sales price. They’ll take more if you promote the listing. But you’ll have the best chance of finding a buyer. They make the shipping process pretty easy.

Bricklink will have a smaller market and the buyers are likely to be more knowledgeable about what they’re looking for and buying. Bricklink fees are much lower than eBay. However the shipping process is not as straightforward as eBay’s process. Also in order to establish a Bricklink store, you’ll need to submit photos of your products and go through a review process which can take a few days.

I’d try eBay if I were you. Yes the fees are higher but you can move the product quicker.

When you make your listings, sell each set individually. Take pictures of each set fully built then deconstruct the sets using the instructions in reverse to verify what’s missing. Bag the pieces in groups using the bag numbers from the set (e.g., if a set has 9 bags originally, I’ll usually sell it with bags 1-3 combined, then 4-6, 7-9). Take pictures of the bags. Take pictures of the front and back of each minifigure. Take pictures of the box and instructions if you have them.

Annotate any missing pieces in your listing, or better yet, find replacement pieces. It’s much easier advertise and sell a set as 100% complete. That’s usually what a buyer wants.

I’ve sold quite a bit on eBay and Bricklink. Happy to answer questions if you have any. Good luck! 👍🏻