i was reading this thread and i realised i had to help.
anyways, when buying a modified ps2, make sure you know what chip you are getting. there are many out there, so choose the right one. have a look here... http://www.videogamesindonesia.com/labs/july/jul16/jul16ps2bg3_01.htm
ask the shopkeeper what chip is in the console. there is a chance he might lie.
so get him to try the chip out. test ps2 cds, dvds, originals, ps1 games, dvd movies, etc. see if they run like they are supposed to (this depends on the type of chip).
buy from a shop with warranty...i know this is hard but not impossible.
if they are willing to open the ps2 in front of you, that's even better because you can then verify the chip (note: you will need to know how the chip looks like).
so do lots of research on the net before you buy!!
of course, buying an unmodified ps2 and original games is always the best way to go.