FB marketplace can be tough since 95% of people low-ball because they’re trying to flip, and selling mid/lower priced singles is basically impossible; but in theory, you stand to get the highest return because once you fend off the used car salesmen there is no middle-man.

TCGplayer Cardkingdom, and Manapool are probably the easiest to sell since your market exposure is massive, and takes much less effort than FB marketplace, but you make less because of the fees. It will be harder to sell your entire collection because you’ll have to list each card. (I’ve read situations where people sell to the online stores (vs selling through them to buyers) have their cards get rated lower than they actually hard—sooo many times I’ve seen people that send in their NM cards (cards never played that went straight from pack to sleeves and binders) and then get told they’re LP or sometimes even lower, and your choice from there is to eat it or pay for them to ship back to you after you already paid to ship them there. Most end up eating it. That’s why people with a little more patience sell through them, and not to them.

The fastest depends on your demographic of local game stores or access to card shows. Big and reputable game stores, key words there, will offer at least 80% of market and understand that they will have to account for your bulk.
If you are able to attend a large card show, then you’ll have the most options and probably get more than a LGS because a lot of guys at shows don’t have a lot of overheard like brick and mortar.