As far as lessons we have to learn while playing Magic going, playing to your outs is one of the most important. Top decks that save games only even happened because the pilot put themselves in a position where if they drew their out they could still win. In finance an out is a much different thing. Your out (or outlet) is your method for converting cardboard to cash or open positions to closed ones. Of course, you need to shop around each sale to really get the very best value, but having outlets prepared for various types of cards is very important.
Often times, people will ask me, "What's the best way to sell XYZ?" My answer is most frequently EBay, but there's a handful of caveats here. It's just as important to consider why you are selling as it is what you are selling.
EBay is generally your best bet, so long as you aren't in any immediate rush for your sale (either because you're short on funds or because you expect prices to change promptly). EBay promises you that players will pay fairly close to retail values, but you'll be stuck paying some fees both to EBay and Paypal.
These fees add up to approximately 10+% of your sale price, and this adds a significant dent into your bottom line. EBay is also risky in the sense that it is not a quick method of sale. During the time it takes for your item to sell, things could change and you may end up losing some money. So if I've been speculating on a card, and I feel now is a decent time to get out, I'll typically recommend EBay if I don't think there is a risk in waiting for the auction to close.
If you don't have time to wait for your product to sell on EBay, you also have the option to sell your items to your local LGS or an online store. You're going to get a much lower value for your stuff from a retailer, but you get your money immediately. To sell to your LGS you'd need to be fairly confident that values are dropping on the cards you want to sell before you could sell them any other way.
Selling to online buylists is a little better on the wallet as you can shop around more buylists online and find the very best value on each card you you want to sell. This does leave you paying your own shipping costs unfortunately. This is a middle ground, finding dealers or online stores that give you good buylist prices are important to have lined up in advance, so when you need to make a sale you aren't wasting precious time hunting down options. Standard cards more frequently fall into this category as their pricing tends to be very volatile compared to older formats.
I'm lucky enough to have a local EBay dealer who gives very good Buy prices on stuff he turns around on EBay. This is most frequently how I get rid of my cards, if I don't want to wait for EBay myself. I get the benefit of a LGS or online retailer in the sense that he'll buy nearly anything, but I also get some of the value from EBay because it provides him a market to sell. He even gets to sell at above average pricing due to his volume store level on EBay and high ranking in the user rating system. For me, this is often the perfect mix of both worlds, as I always get great value, without any time risk or loss.
If you don't know any local EBay dealers, try shopping around EBay a little, and use the search filters to find out if there is someone local that moves items at a decent volume. Network with that person and you're all set. If your local community doesn't have an EBay dealer, maybe there's an opportunity for you to become one. I wrote an article about how to run an EBay store about a year ago, which you can find here.
The reason you'll want to have your outlets lined up in advance, is you don't want to hesitate when you have a card you need to get rid of before a price change. Being first to make a move, even after an announcement that triggers your decision, will often be enough to guarantee the buyer will still take what you have. If you act too slowly, however you give other people the opportunity to beat you to those sales before buylists are adjusted. Some of the most common times I'll go to EBay with my sales are during Pre-Release. I'll set 3 day auctions at the end of the Pre-Release weekend, so that they end well before the release date and you'll still have a chance people will fight over what you've got. Especially if you got a desirable foil.
Because my local EBay dealer gives good value on most things, there are fewer times I sell on EBay, but bulk is something to consider. Often times what I would make off of my bulk, minus the transaction fees, it makes more sense for me to go to my dealer anyway. If you don't have that luxury, having some pre-made listings saved somewhere will go a long way to make your life easier when it comes time for posting.
No matter what you're selling or why, be smart about it. You don't want to bleed value by underselling your stock, but you can't afford to bleed time either by waiting for lengthy auctions, shopping around on buylists, or waiting for someone to contact you from MOTL. (As a note, I think MOTL is a great place to sell and buy cards, however speed needs to be very low on your priority list). Have your outlets prepared in advance so when it comes time to make a move you can efficiently find the right outlet promptly.