In H2 the generated schema works:
[main            ] DEBUG  - 18:27:01,763 - org.hibernate.SQL: 
    create table Item (
        id bigint not null,
        name varchar(255),
        SELLER_CUSTOMERNR varchar(255),
        primary key (id)
    )
[main            ] DEBUG  - 18:27:01,769 - org.hibernate.SQL: 
    create table USERS (
        id bigint not null,
        customerNr varchar(255),
        primary key (id)
    )
[main            ] DEBUG  - 18:27:01,769 - org.hibernate.SQL: 
    alter table Item 
        add constraint FK_76d41162ea614944b8333c7c440 
        foreign key (SELLER_CUSTOMERNR) 
        references USERS (customerNr)
[main            ] DEBUG  - 18:27:01,775 - org.hibernate.SQL: 
    alter table USERS 
        add constraint UK_8a5ec890d05a4545a1e9660dea0 unique (customerNr)
On Oracle this fails, as the foreign key reference is actually checked before the UNIQUE constraint is generated.
http://www.techonthenet.com/oracle/errors/ora02270.php