I noticed that when some people sell a website, they say something like "offering free hosting for 30 days" - do they do things like give you access to the ftp server for the website, or do they simply just give you a log-in info for the site and that's it?
A few would probably put a few limitations on what you can do, but for the most part you'll normally find you have full access for the trial period.
Why not ask the host in question? Only they will know the correct answer!
You should just get your own hosting and know how to use it. That way you wont get ripped off.
The short-term hosting is usually free - avoiding any rip-off issues
Every auction/seller is different so make sure you ask them before you buy. It's also worth asking if they will transfer the site to a new hosting provider.
I recommend that you get the site transferred to a new hosting account as soon as you can, that way you have full control over the site.
If the seller won't transfer the site, make sure you can get access to cPanel. This will give you access to files and any database the site uses.
This is usually not a scam. It is usually just a sales offer to get you to start paying money. Just like timewarner offers free hbo for 6 months. Its good for a short period of time but it may screw u.
Your question is probably one better suited to the Host Your Site forum. There is plenty of information in the sticky threads (the pink ones at the top) and if you need more, then you could consider starting a new thread over there.