One of the statutory eligibility requirements is to have a "round trip ticket." See 8 USC 1187(a)(8):
(8) Round-trip ticket
The alien is in possession of a round-trip transportation ticket (unless this requirement is waived by the Secretary of Homeland Security under regulations or the alien is arriving at the port of entry on an aircraft operated under part 135 of title 14, Code of Federal Regulations, or a noncommercial aircraft that is owned or operated by a domestic corporation conducting operations under part 91 of title 14, Code of Federal Regulations).
"Round-trip ticket" is defined by regulation. It's a bit broader than the usual meaning of the phrase. See 8 CFR 217.2(a):
Round trip ticket means any return trip transportation ticket in the name of an arriving Visa Waiver Pilot Program applicant on a participating carrier valid for at least 1 year, electronic ticket record, airline employee passes indicating return passage, individual vouchers for return passage, group vouchers for return passage for charter flights, and military travel orders which include military dependents for return to duty stations outside the United States on U.S. military flights. A period of validity of 1 year need not be reflected on the ticket itself, provided that the carrier agrees that it will honor the return portion of the ticket at any time, as provided in Form I-775, Visa Waiver Pilot Program Agreement.
However, there are some relevant restrictions. See 8 CFR 217.2(c):
(c) Restrictions on manner of arrival -
(1) Applicants arriving by air and sea. Applicants must arrive on a carrier that is signatory to a Visa Waiver Pilot Program Agreement and at the time of arrival must have a round trip ticket that will transport the traveler out of the United States to any other foreign port or place as long as the trip does not terminate in contiguous territory or an adjacent island; except that the round trip ticket may transport the traveler to contiguous territory or an adjacent island, if the traveler is a resident of the country of destination.
(2) Applicants arriving at land border ports-of-entry. Any Visa Waiver Pilot Program applicant arriving at a land border port-of-entry must provide evidence to the immigration officer of financial solvency and a domicile abroad to which the applicant intends to return. An applicant arriving at a land-border port-of-entry will be charged a fee as prescribed in § 103.7(b)(1) of this chapter for issuance of Form I-94W, Nonimmigrant Visa Waiver Arrival/Departure Form. A round-trip transportation ticket is not required of applicants at land border ports-of-entry.
Your bus ticket to Mexico fails to meet the requirement on two counts. First, the destination is "contiguous territory," and, second, the bus company is not a "participating carrier."
Immigration officers may overlook this requirement, as may the airline, but then again they may not. I wouldn't risk it. You can either buy a fully refundable ticket and refund it later, or you can apply for a B-2 visa.
Of course, a restricted round-trip fare may be less expensive than a one-way fare; if you get such a ticket, you won't need to worry about this requirement because you will fulfill it. Thanks to Aleks G for pointing this out in a comment.