Well.
Ebay is NOT bad. Ebay is like wild wild west - if you manage to know the rules + tehnical CCTV expierence and get a little luck - you may succeed and really cut some costs.
I buy on ebay frequently and i cant really rant on ebayers. You must know the rules and tehnical details.
You can build pretty good home monitoring with 4 cams and PC dvr under 200$, YOU cant equip fort knox with ebay stuff on second thought. Ebay is good for small and not-so-professional projects, and there is anyways risk too. Plus. Ebay is full of cloned DVR cards and software. Be aware. Talk to sellers. Ask them, take a look on their history.
So conclusion:
1) YOU need to know ebay rules, tips and tricks
2) YOU need to know atleast basic tehnical details(forexampl. what is diff. PAL with NTSC or what is lux or view angle)
3) NEVER buy ALL equippment on ebay, just some parts
4) think twice