The Garage Sale is a pretty big event every year. The stuff they sell varies, but you should know the predominant clientele is always youth hockey teams, recreational hockey types, etc. because it's an opportunity to get lightly used or new-in-package high-end hockey gear for a tiny fraction of what it should cost. A lot of people go and buy their gear for the year in one morning. There's tons of sticks, gloves, padding, skates, etc. etc. Entire teams and people start lining up the night before and take turns camping out on the chairs while others sleep in the car. I went a few years back, got there at 5AM, and I was halfway around the back of the building. By the time doors open, the line is usually stretched all the way around Yost and then some.
There's plenty of pretty cool stuff to be had, though, even if you get there late. I picked up a practice jersey (turquoise-first line), one of the team-issue Nike track suits (most comfortable clothing I own), and a couple other random things the time I went. They sell, literally, everything. The goalie masks were out there for upwards of $500, they were selling game helmets for $75 (that was the first time that happened, not sure if it continued), practice helmets, team-issued gear (even underwear, socks, etc.), the whole gamut of stuff.
One of the popular items is always the winter ski jackets the team gets if they have to travel out to Fairbanks. There's also practice jerseys, which are color-coded depending on the line. Goalie jerseys are black, first line is turquoise, second line is blue?, third line is ?maroon?, injured jerseys are red, defensemen are gray. I think I paid like $30 for mine. Prices are generally pretty reasonable on stuff, except for the obviously collectible things like goalie masks.
All in all, it's a very, very fun event, but it's a lot of waiting in line to fight through hordes of hockey players looking for gear, and from what I hear, the haul varies from year to year. It's never made clear what they will and will not have, so you may wait in line for hours and come out with nothing.
Probably worth attending once just for the experience.