Second hand tools are fine. But, be sure that you are getting a good deal. Power tools from a pawn shop, for instance, are an awful deal.
There are two schools of thought in buying tools. The first kind of buyer will get the cheap stuff because he knows that he will rarely use it. The second kind gets the expensive stuff because he wants it to last forever. I like a combonation of the two. For 'hand' tools, I buy the cheaper sets. You can find wrench sets, drill bit sets, screwdriver sets, etc... All of these will be prepackaged in thier own plastic case, for easy storage. They are handy to have and don't cost an arm and a leg. My socket set, for example, cost me $20 and has handled 95% of my needs. (I had to buy a seperate socket for my spark plugs.)
For 'power hand tools', I invested a bit more. DeWalt has a set of 6 or 7 tools (drill, impact wrench, vacuum, flashlight, reciprocating saw, angle grinder, trim saw) available for about $500. It comes with 3 batteries and a charger, as well as a heavy-duty duffle bag. This is a GREAT deal. The batteries alone are a $300 value. These are 18V rechargables, and they will do most any job around the house. It's a big purchase, but one that will last you a long time.
Other things you might want: miter saw, table saw, air compressor and nail gun....the list is endless. In general, I find that people tend to want a lot of money for used tools, so be careful. I would rather buy a brand new 'off-brand' (Ryobi, Rigid, etc...) than spend the same on a used 'high-end' brand. There is a certain comfort level I have when I buy a tool new. I know it hasn't been abused, and if you are only doing work around the house, they should be fine. When you buy a used tool, make sure that it funtions as it should. A saw shouldn't wobble, for instance. Also, smell it. If it smells of 'ozone' after running for a moment, then the engine is burnt up.
Good luck.