We have a timeshare week down in Cabo that was purchased at a moment of weakness during our honeymoon. Its nice having that 'built in' vacation that we can rely on, and its a really nice resort (Playa Grande). As for taking the tour / getting the sales pitch, it sounds like you've got the right attitude. You definately have to weigh the "cost" of the time and hassle it takes vs the benefit. Our trip down to cabo this year we were adamant that we weren't going to do any timeshare tours, and we didn't. I even turned down $500 cash + a dinner cruise. My vacation time is MUCH more valuable than that.
I don't recommend timeshares to any of my friends, but for a lot of people, they're a good value if used properly. We cancelled our RCI membership, and can no longer trade our timeshare for other resorts, but thru our resort we can bank, borrow, and buy weeks.
I've never tried buying a week off of ebay, I'll have to check that out!