I bought my latest boots at REI and they are Merrell's. Good medium grade day hikers, costing about $120 something. Mind you, I'm no expert on what is best, but I can offer this:
Find out through research what holds up. Make a list and then if you can, go to Cabellas, Bass Pro, REI, etc and with list in hand, try them on. If not, you take your chances ordering online. Just make sure you have plenty of time and they have a good return policy.
If you can scrape together $100 for a pair of boots, then don't pass up the $140-180 pair you really want. Every step you take will remind you of your choice and believe me, a 15-20 mile hike in one day is hard on your feet even with good boots.
If your hiking needs are rocky ground, get all leathers. If you mainly do Urban hiking/cacheing, then go with something more tennis-shoeish, but no low quarter shoes. You need GOOD ankle support.
First and foremost, they should be water proof and have a good tread pattern on them. When I was in the Air Force, I wore my Combats everywhere and wouldn't have it any other way, but seriously, for geocaching, there are better shoes/boots.
I would not consider buying a used boot off of ebay.
Shoes and boots are very relative to the wearer. However, if the boot has an excellent history/warranty you can not go wrong (after actually wearing them). All this is just my opinion, YMMV. BB