Depending on what you are looking for, I think used bikes can be a great deal... providing of course they have been tuned up by a bike shop... not simply a weekend warrior. As for buying parts for the bike, "yes" you still can buy parts and upgrade the bike as needed. The Rockhopper has always been a great bike, and probably worth looking at. The generally have 3-4 diferent models of the Rockhopper.... even the base model would be probably around $350 new. The '91 model more than likely would have a steel fork(as someone else said) but a great bike nonetheless. If the Rockhoppers were not a great selling bike, they simply would have changed the model by now... that in itself should say something.

From year to year, technology becomes that much more aggressive and over time that gets past done to less expensive bikes... the high end bikes have always are the starting point for a lot of this. To give you an example, if you had a bike that was three years old and you paid $500 for it, today you could buy that same bike with the same technology for $300-$350... obviously it's that way with everything.. from cameras and computers, tv's .. you get the picture.

As for looking on Ebay, I know that many people have had good luck there... I on the other hand have seen some terrible cases... from cracked frames, stolen bikes, and even manufacturers that have bikes that didn't pass stress tests and they sold them on Ebay under a fictiousious name.... So my word of advice is be careful... if it seems to be too good to be true.. it generally is!

Lastly, probably the one of the most expensive things that can go wrong with the bike are the wheels... so I would definitely do some research on that.(Rockhopper)

Have fun, and hope that you find just the right bike!