With an already severely compromised immune system, and the huge variation of blood borne diseases, there is nothing much helpful from doing this in this way.
The existence of any antibodies in the bought blood are not going to be helpful in the creation of the infected person's own antibodies.
It is more sympathetic magic than anything curative.
As a vaccine, that is a different matter, but you are giving yourself a disease (or several) that is likely just much better avoided.
There is one paper which shows that in some cases the blood transfusions may actually help to survive:
http://jid.oxfordjournals.org/content/179/Supplement_1/S18.long
However, the method was tested only with 8 patients: "because of the small number of patients studied and the lack of control subjects, we cannot conclude that the neutralizing antibodies in transfused convalescent blood improves the outcome for EHF patients."
I'm guessing that this study was not done with blood bought off the street.
Buying and transfusing blood from the "black market" (without even a SR-like rating system)?
That is desperation.
They are now testing blood transfusion with an NBC cameraman: "Ebola survivor Kent Brantly has donated blood to a fellow patient for the second time. This time, Brantly, a physician, donated blood to an NBC cameraman who also contracted the virus while working in Liberia.
Although there's no proven therapy or vaccine for Ebola, doctors hope that blood transfusions from survivors will provide current patients with antibodies that will help their immune systems fight off the virus."
http://www.usatoday.com/story/news/nation/2014/10/08/camerman-ebola-transfusion/16906983/
