Sound advice for the new member.
Depending on location, 20% is a great starting point for discounts. Everyone now and again, retailers will offer BOGOs to get the overall discount into the 30%s and into the 40%s if the sets are already reduced prior to the BOGO. Exclusives (at least in the US) is seldom discounted anymore so using Amazon points, eBay bucks, shop@home points etc... is one way to create your own discount.
Selling Lego after EOL can be risky . You have refreshes to worry about, missing elements, condition, the manual, picking the right sets, etc... However, I found selling sets my son and I don't want anymore is a wonderful way to roll the profits into "dream sets" like 10196,10181, etc...
I always advise folks to buy what you want to build and think about the selling as an afterthought.