IBM Corp is making life so tough for operators in the used computer business that Oak Brook, Illinois-based Sentinel Computer Services Inc is scaling down its Reconditioning Operation and expects to be out of the business altogether by December 1994. In the meantime, the facility will be staffed and operational and will meet all current commitments and will continue to offer auditing, reconditioning, technical and storage service on a limited basis. Sentinel blamed the increased competitive pressure created by IBM and IBM Credit on the leasing industry as the principal factor in the decision. Sentinel also handles DEC, Wang and Sun kit.