My $.02... We bought 50+ lots from shopgoodwill.com up until a year ago, now just occasionally buy for my collection.
Pros:
* Variety is huge and nearly all sets and figs you'll find are retired.
* It was a great introduction in to brick picking. You will definitely learn your parts, your sets, your Lego eras. If you're starting out with more time than money, and you LOVE sorting, inventorying and assembling Lego, the returns can be decent on a % basis. A typical buy was $125-150 for a 30# box, buy another $25 of parts and manuals and sell on eBay for $6-800. It worked out to a decent stay-at-home hobby/gig for about $15/hour. Open a beer, turn on the TV and have fun.
* If you're a collector and want to build cool sets that you would never otherwise buy, you can. I've had old castles, pirate ships and a ton of large SW sets go through my hands. Now I'm only collecting vintage late 70s and early 80s. Latest addition from the site is a very nice 956 auto chassis with a great original box & a 375 yellow castle.
* If you're running a parts store this is an awesome way to get a huge number of unique parts.
Cons:
* It is very difficult to buy sealed sets as they often go for above retail. Some people justify their purchase prices because "Hey, the money's going to a good cause.", making it hard to win some auctions. Of our 50+ buys only 2 of them were for sealed boxes to flip. The rest of the cons are based on buying partial sets and parting them back into complete sets...
* You HAVE to study to be able to identify parts, figs & sets to bid properly. Or you will overpay or lose to bidders that did their homework. You may get lucky, but that's gambling.
* Scalability is zero. It is time-consuming from one end to the other. Once I got into selling new sets it became sooooo painful sort, inventory, place and wait for BL orders, photography... yada. And it's really nice to list 10 or 20 of an item rather than 1 at a time.
* You have to start your own private parts store. I amassed 100,000+ parts, 80% of which are part-sorted (not color-sorted), but I don't have the time, space or desire to start a (real) parts store. If you don't have a decent (sorted) parts supply of your own, buying individual parts will cost a LOT of money.
* Unscrupulous listers, i.e. photograph a 15# box of garbage and show a few HP figs and sand green slopes sprinkled in to make you think the box has some value, just to see that you paid $80 for 1# of HP Lego and 14# of bricks.
* Ignorant listers, i.e. someone donates a beautiful 30# SW collection and the lister pulls all minifigs into another lot, and then dumps the parts randomly into 3 lots... You'd better buy all 3 lots, and now it's time to buy figs. BTW, the lots often all end within 30 seconds of each other : P (PS - some listers are AWESOME and you will quickly see what locations to avoid)
It was a good education, but you really have to enjoy the tedious process. Ultimately my desire to ramp up the business pushed me out of buying from them. But it is pretty cool to pay $65 for a pile of bricks and an empty 4842 box (confirmed by the lister that it's empty), to get it home and find complete a 4842 & 4867 inside the box : )