Buying a timeshare from an existing owner

What are the trade-offs between buying from Marriott and buying from an owner?

ONLY one thing, the abilty to trade your TS usage for Marriott Reward Points. Period

If I buy from an owner, can I still bank weeks, trade etc.?

A friend showed me a site that they use called Interval International. Would I be able to use that?

Yes absolutely.