This is an excellent thread. Way too many people concentrate on buying and don't ever try out selling before they are just too deep. I am one of these people and I have long since learned my lesson (I think). For the first year and a half all I did was buy. I bought up over 1000 sets during this time. I would say that I was addicted (still am).
The point came where because of money and space it was time to offload some of my sets. In order to buy more I would need to start selling first. So in January of this year I started selling. This was an eye opener. I know everyone says that it takes time to list, sell, package, ship, etc but you really don't understand until you do it yourself. It is very time consuming. You also have to expect there will be problems. Lost packages, buyer returns, etc. This all eats in to your profits and time. I know a lot of people say large sets are the way to go and I agree to a point. Just know that those sharks on eBay/Amazon looking for sets to scam people on aren't looking for small inexpensive sets. I've got burned a few times already and it is always on large sets. A mint condition Imperial Shuttle returned banged up and opened, a Robie House returned destroyed after FedEx left the package in the rain, a AS returned because the buyer "bought it for someone who didn't like LEGO", a Robie House and AS on the way back now for a mystery reason, 0 feedback buyers everywhere, etc. Some of these can be avoided with experience and others are just part of the selling process you have to deal with.
And here comes the big one everyone is warned about over and over but never really understands until they sell.... FEES. The fees are much more of an impact on your bottom line than you may think. Make sure you find a good fee calculator online and just play with it a little and enter some values in for sets you have. See what you must sell at to get a certain return on your investment. This is easy and takes just a couple minutes but it will be an eye opener for some who haven't even thought about the selling side of this yet. Make sure you add in estimated shipping costs and add a little for supplies. I know most people don't think about tape, boxes, paper, ink, etc but it adds up very quickly when you start selling at a higher volume. As an example I have spent $40 this year just on packing tape itself.
I made the mistake as I am sure a lot of others do of thinking if rough times come or I need the money to reinvest I will just sell a bunch of sets and be fine. The truth is it takes a lot of time for things to sell. Like others have posted its not like selling stock. It will take months before you can sell a large inventory of sets. If you need your money fast you will be basically giving your sets away to sell them fast. Many times I have been the lowest price and still had to wait weeks for the item to sell.
To sum this all up I am still learning but I feel much more confident that I have gone through the selling process and know what is involved. It will actually change your investing and buying decisions. It has drastically changed mine and now I am making more money and spending less time because of it. Since I started selling in January I have sold almost 600 sets totaling just over $37,000. This has allowed me to grow my investment to over 2100 sets and make much better investment decisions. If anyone wants the exact numbers on profits, fees, etc on those 600 sales you can PM me.
Buying is only half of the investment process. Make sure you try the other half before you get in too deep.