There's no single "best" way to sell LEGO — it depends on how much time you've got and how much hassle you're willing to take on. Here's an honest run-through of the options, so you can pick the one that fits.
Car boot sale
Good for: a quick clear-out where you don't mind a low price. The catch: you'll likely get very little for a tub, spend a morning haggling, and bring home whatever doesn't go. Fine if convenience isn't the goal.
eBay
Good for: individual rare or sealed sets, where you can attract collectors and get a strong price. The catch: for a big mixed collection it's a lot of work — photographing, listing, answering questions, dealing with no-shows and returns, then packing and posting each item. Fees eat into the total too. The price per set can be high; the effort is higher.
Facebook Marketplace / local selling
Good for: shifting things locally without postage. The catch: strangers arranging to come to your home, lowball offers, time-wasters, and a lot of messaging. Works for some, grating for others.
Online buy-back service
Good for: selling a whole mixed collection — boxed, built, incomplete and loose — quickly and in one go. How it works: you get an instant price, send everything free with one label, and get one payment. No listings, no photos, no strangers, no sorting. The catch: for a single rare sealed set, a patient eBay seller might net more. For a typical family collection, the convenience usually wins.
So which is best for a parent clearing out?
If you've got one valuable sealed set and plenty of time, eBay can pay off. But if you've got the usual mix — a tub of loose bricks, some used sets, the odd sealed box — and you mainly want it gone with minimum fuss and a fair price, an online buyer is almost always the easier route. One pile, one transaction, one payment, even with the incomplete stuff.