There is a pretty flimsy paper gasket behind that sheet metal axle shaft cover that is probably the source of the leak- - - -rear end lube, not brake fluid. See if the seller will take some pictures of the inside of the dual wheels- - - - -a brake fluid leak will be dripping down the inside tire as it runs out of the brake drum.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!