Lots of things. Is the boat complete? Should include Mirage Drive, daggerboard, seat, paddle, etc. Look carefully at the Hobie site to see all that's included. Other accessories (wheels esp) with the price?
Everything should be operated. Unfurl / furl sail, rudder up / down, look inside all hatches (water), check aluminum parts for corrosion, condition of bungees, pouches (easily replaced). Try to inspect underside if possible for obvious dents, heavy skid marks.
You may have trouble getting things apart. Better have some WD-40 if you're planning on disassembling.
Check serial #'s or purchase receipt to verify ownership and model year (last 2 digits of #). Unfortunately SERNO is on bottom near stern. First ones were '07; nice upgrades in '08 if you have a choice.
A neighbor bought one used recently from a dealer. It had a big chunk missing out of the furling spool -- he didn't notice. Look everything over carefully. Good luck.