Some ideas (I would ignore tax value personally):
The longer the home has been on the market (3+ mos) the less firm on price the sellers will be...
Maybe your realtor can find out why they are selling, more desperate means more negotiating room...
Ask your realtor what a comparable property (in good condition) would sell for (or get a CMA for the area). Put a rough estimate on the repairs your are going to make (estimate high and include your blood, sweat and tears, not just materials). Using those numbers you can get an idea of what the home will be worth when you finish fixing it up and what it will take to get it there.
Ex. Comparables are $70k, repairs are $15k, maybe offer $55k to make up the difference.
Finding online information about previous sales will be tough in your location, usually the larger municipalities offer those services, but they are public information and if you want to look hard enough you can probably find out (maybe realtor can ask?). Knowing is a powerful negotiating tool.
Obviously an offer lower than the asking price may get rejected so you are playing a game. Be prepared for them to say no or counter offer.
If you are in a really powerful situation, offer less than your 'number' and see what they say...