If they're blacklisted, you might be safer 
saying, "Hey, there! I was admiring your gallery 
and realized you owned the cel that belongs to my 
genga. I was wondering if you would like to 
reunite them; I'd be willing to offer it to 
you..." and so on. If they send the money, and it 
clears, send the genga along. If not, you didn't 
lose anything.

*Since* they're blacklisted, it would probably 
not be a good idea to say, "Hey, there! I was 
admiring your gallery and realized you own the 
cel that belongs to my genga. I'd love to buy it 
from you!" You might get bitten that way, unless 
you know of anyone who can independently vouch 
for this person's trustworthiness, blacklisting 
aside.

Good luck,
-Cres