When dealing with someone of unknown reputation, i would start with a small value, then if that goes well, double, rinse&repeat 'tll you bought/sold all you wanted
That's not realistic if you're putting up a limit sale or buy. the system matches you up with whomever has a matching offer. Then you just have 2 options: take it or leave it.
If you like that kind of gamble, you can try this little game I stumbled across: