we pretty much buy out all of a certian item hat is at its lowest price, wait a few days and then dump it, meaning we have ALL the items for a few days which raises the price a lot. don't join if you are going to sell the items early, you'll just get kicked. i think u can apply at the begining of each month, i dont remember
everyone thinks certain merch clans are better than others. all depends on who u ask.
the only advice i can give you, is be ready to (possibly) lose some money. it can happen. never invest more than half your total cash just in case you miss the announcement that it's time to dump the item.
you can make tons of money merchanting, but u can also lose money doing it.