Good plan buying used to start. Buy new boots if you can afford them though...
Tough to guess what the board is worth without know models, condition etc. A 3 year old K2 setup could be worth anything from $100 to $300 or so if it was pristine and an expensive model to start with.
Also, make sure the board is going to be the right size for you. The model isn't that important to start, but size is. Too small and you'll reach it's limit very quickly, too long and it'll make learning hard.
Here's what you want to check for on the board. Don't be shy when you're checking it:
1) Look all over the base. Look for gouges that are so deep you see wood underneath. Also look for odd colours or clear sections. These are indications that a base repair was done. Nothing wrong with that, but I'm not paying $300 for a board that's been base welded.
2) Check the edges, they should not be separating from the base anywhere, and shouldn't have major gouges in them either. Dull edges can be sharpened as long as the board wasn't detuned for rails.
3) Check the topsheet, some minor cracks in the topsheet are okay, it just happens when the board flexes. Some brands are worse than others for this. I had a Burton T7 that spider cracked almost right away. Not a big deal. Make sure there are no BAD gouges that expose the core.
4) Check the binding inserts (threaded holes that the bindings screw into). If the bindings are mounted, try reefing on them fairly hard in a twisting motion to see if the screws are lose. Somebody who takes care of their gear won't have lose binding mount screws.
Check the bindings:
1) Look carefully all over both frames/baseplates to ensure there are no cracked sections.
2) Check the highback for cracks.
3) Check all straps, make sure the ladders are in good shape (the teeth shouldn't be worn out, though this is an easy fix).
4) Look for odd looking hardware. If something was replaced with non-factory hardware it could have been a shoddy fix. Original looking hardware at least tells me that if a repair/replacement was done on straps or parts, they used the factory stuff.
Overall there are only a few things that would make me not buy a board. Edge damage, delamination, or messed up inserts aren't really worth fixing unless the board is almost free. Other issues such as a scratched topsheet or lots of minor base gouges aren't a big deal, but they should bring the price down. Bindings need to be in real solid shape for me to give any value to them.
Hope that helps...