I think it's because it would be very easy to make a 'home away from home' once you owned one of those lots, and buy all the stuff you want. Although ultimately, after some testing, I really don't see that it would be that much of a help anyway.
As for collecting, can our sims collect things but not place or sell them? Some things are needed for opportunities (sorry to push the issue - but they were allowed in the previous challenge) and can be used in careers.
What about wedding presents and gifts recieved as celebrities? Are we free to sell them? (or place them as one of our 4 items)
Are we allowed to sell stuff that we steal?
What do we do with career rewards?