Megan McArdle ably bats back the notion that this hypothetical is supposed to be a hard question for libertarians. Yeah, incapacity to contract was recognized as a defense at deep common law -- i.e. when society was much more classically liberal than now.
Also, there's one more distinction between the very poor and the elderly wealthy victims in the hypothetical. Merchants are often reluctant to give credit to the former because the very poor are bad risks. To lower their downside risk, they charge higher interest rates and so forth -- the practices that are sometimes characterizes as exploitative. See generally. Without the ability to charge higher rates to protect themselves from risk, some merchants won't make risky loans at all. And the poor can wind up worse off. That's not the case in the example of the wealthy elderly person. In that regard, the two scenarios are actually quite different.
There's another post about why this scheme, as well as being immoral, would also be stupid.