Learn English – word order; each the grave vs the each grave

Confer is monotransitive: it takes only a direct object, not an indirect object, and the recipient of what is conferred must be expressed with a preposition phrase, usually headed by on or upon, although to and with are found in very old writings.

^okThe university conferred a degree on him. BUT NOT
^∗ The university conferred him a degree.

Consequently only the direct object, the degree, may stand as the subject of a passive construction.

^okA degree was conferred on him by the univesity. BUT NOT
^∗ He was conferred a degree by the university.

If you need to make he the subject of a passive construction, you must use another verb such as grant or award:

^okHe was granted a degree by the university.
^okHe was awarded a degree by the university.

Of course you may make he the subject of an active verb such as earn

^okHe earned a degree from the university.

ADDED: Arrowfar points out that your variant be conferred with has appeared in a number of Irish, Australian and South Asian sources: He was conferred with a degree. The active version is much rarer however; I take this to be an error derived from confusion with the use of confer with in the sense have a discussion with.

This question relates to two debated points of English style: Who vs Whom (warning: silly but excellent explanation) and whether it is OK to end a sentence with a preposition (short answer: "yes — depending").

"Whom" is not common in informal (and American) English. Also, in informal English, you may end a sentence with a preposition. Consequentially, if you are going to use "whom", you already sound more formal and probably should not end a sentence with "to".

So a question such as

To whom are you speaking?

is natural, but formal. Most would instead say (some variation of):

Who are you speaking to?

By the same logic the natural, informal version of your example would be:

Who does the Islamic state sell their oil to?

As with many things, which you use depends on your audience. It can sound odd to be formal with friends and coworkers, but it can be similarly inappropriate to be too informal in business or professional communication.

However even this varies considerably in different parts of the world and different industries. As an American — or, more specifically, a Californian — I only use "whom" as an affectation to make me sound particularly formal (or British). Since I work in technology, there is no need for me to use it professionally, as technical communication tends to be informal. The only place I might use it is with legal documents, formal letters, or invitations to formal events.