it wouldn't be terribly expensive to get it fixed. depending on what is wrong of course. i'd guess a few hundred bucks to have someone go through it completely, find the leak, check the master cylinder, bleed the lines, refill, etc.
i just had a new actuator assembly and master cylinder installed on my trailer. lines blead, brakes double-checked, etc. it was several hundred, but i had to buy the entire actuator assembly clear back to the break-away from MC, have it metal-etched, painted, clear coated, etc.
i would not want to trailer any great distance without brakes. why not somehow make it a condition of the sale? not sure what state you're in, but many states require brakes on any axle that comes in contact with public roadways.