I bought a cat tree on ebay... small (non armorkat) tree...42", w/ a cubby on bottom, duel poles supporting a single shelf, then a single pole supporting the top shelf.
New, in box
Easy to assemble
I got it for $25 (5 for the tree, 20 for shipping! ) (I have since seen the tree for $20 and shipping for $20)
carpet is thin and there are some exposed staples on undersides of shelfs and in cubby
Not very stable
Shelfs are smaller than I thought they would be (the ad didn't give dimensions)
If you are going to buy one off ebay...basically the only downside is that you can't see/feel the tree for yourself to see how stable it is or how good quality it is. I'd suggest going w/ a name brand that lists specifics in the ad like shelf sizes, max. weight per shelf and total, etc. And by all means check the seller's feedback.
One more note...look for a tree w/ multiple support posts per shelf (or a tree with real wooden supports) to help stability-wise.
For myself...the tree has lasted OK (I've had it for about 10 months now)..But that's about it. I really need to get a new one and I'll more than likely be getting a lotus or similar that has replacable shelfs and involves wood (so isn't as rickity). Sure, it'll cost more, but hopefully will last a lot longer.
EDIT: Here's a link to a tree on ebay that looks like the same model as mine: http://cgi.ebay.com/42-Cat-Tree-Cond...1%7C240%3A1318