If someone has a shop that the Rabanim say is hasogas gevul, but he refuses to close it, is it just a problem for the seller or is it also a problem for buying from there?
There is a concept in Halacha of not encouraging somebody to continue his evil ways.
For example: One may not buy from a thief, as this encourages him to continue stealing.
If this is a real case of Hasogas Gevul then buying from him encourages him to continue being Masig Gevul, and it should be forbidden.
Sample source from the Kitzur סימן קפב - הלכות גנבה וגזלה in סעיף ח: