I found a UCS Imperial Star Destroyer 10030 Set at garage sale. It looks complete (90% of it is still in sealed plastic bags). How can I confirm the pieces are all there, and how can I sell a set like this effectively? I'm not currently a seller on eBay.
Welcome! Here are a couple of suggestions:
a. Find a review of this set online, ideally one with pictures which shows all the bags included in the set. This will allow you to verify which bags have been opened (you already know which ones are still sealed).
c. Here comes the tricky part. Find out which steps contain parts from your opened bags, and check if you have all of them. If this is somehow impossible, you have two alternative options:
Open all the bags, build the 10030 and see which pieces are missing, then order those from Bricklink.com and complete your set. List the set as Used, Like New, Built once to validate completeness and you should be able to get a price relatively close to the New price.
Sell the set as Used, 90% Sealed bags, possibly some missing pieces. Depending on how complete it actually is (you won't know) you might get some good offers, especially if box and stickers and such are still present.
To sell LEGO on EBay, you don't need any special priviliges. Just create a listing and sell. However, if you have never used or sold on EBay before, and you have zero or low feedback (<50), starting off with listing a high-value item like UCS 10030 is bound to set off scam-warnings in many of your potential buyers. You'd need to sell other items first before putting this one up for sale. Of course you can always try to sell it via Craigslist, OfferUp, LetGo and other garage-sale apps, but you'll likely get less money for your set (even taking into account that EBay/Paypal will take 12-13% of the sell price and CL/OfferUp etc. take nothing).
I would grab the set inventory from Bricklink and see what parts you may be missing. This will be a time consuming task, particularly if you're planning to keep everything in bags, but this is probably the best way to absolutely confirm that you know what you are looking at. Often, second-hand purchases from garage sales, Craigslist, or similar sources may also contain extra parts or bags not related to the set that you are actually trying to sell, so it can be hard to confirm completeness without actually going through piece by piece.
Once you know what is missing, you can either list it exactly (e.g. 99% complete missing 2 gray 1x2 tiles) or buy the missing parts on Bricklink and sell it as a complete used set.
If you don't want to go through the work of verifying completeness, you can definitely just sell this as-is providing as much detail about what you think is present as possible. Include lots of detailed photos of exactly what parts are present, and you should still be able to fetch a fair price for this.
Regarding selling on eBay, you are able to sell from a brand new account, but this will raise red flags for buyers, particularly if your first sale is an expensive item like this. This may ultimately hurt your selling price. If you have a friend who regularly uses eBay, they may be willing to list this for you.