Parties
There has been a question by Untame about why parties are expensive to make. The truth is, as Xeeroh said,
" impossible for most people to even try to start a party.
That's the point ;P
If people really want a party, they will make it happen, if not they can find one to join. "
So yeah. The parties are to make people work for it, and not because the site wants to get money out of the members.
Retiring.
Items retire over time so that it would make a "fun" challenge for users who want them , collect them. If you wanted to, say, have an Omni hoard, you would work for them, right? Common sense.
Retiring also means that more users will be dedicated to the site and try their best to get the items.
CS items.
These are also not for the site's advantage only. It's for Pat's.
He built this site up and practically relies on it to support himself and his family.
And most of all, the users get to show off when they are out of the shop.