you can usually repair an abs sensor using tinfoil (search for my previous posts if you want instructions) so even if every sensor is faulty its nothing to get worried about. the other possibilty is the abs controller. if that is faulty you can fit a scrapyard part so even that wont cost you a lot to fix. the third option is a wiring fault, also easy to find and fix, so an ABS fault is actually a good thing because you can use it as a bargaining tool to get the price down, knowing full well that you will be able to fix it for little cost.