Lavell Merritt, the Montgomery County minority purchasing officer accused of violating the county's conflict of interest law, has resigned the $25,000-a-year post he held since October 1981.

"It was the best thing to do," Merritt said yesterday of his resignation, submitted Monday to Jerry F. Helvey, acting chief of the county's purchasing division and Merritt's boss.

Helvey said he accepted the one-sentence resignation letter in a meeting both men described as cordial.

In testimony given behind closed doors last week before the county Ethics Commission, Merritt, 55, a retired Army major, acknowledged that he misused his office by conducting some private business from it but insisted that he did not break the county's 1971 conflict-of-interest law.

Earlier this year, the ethics panel said it found a "reasonable basis" for believing that Merritt had violated the law when he asked several firms that have contracts with the county to buy tickets to a fashion show he was sponsoring. The commission said Merritt also abused his office by trying to secure credit from the People's Republic of China and by promoting the Black Leadership Assembly, a coalition of business and religious leaders.

The ethics panel is expected to issue an opinion on the Merritt case within a week. Merritt said he is negotiating with a Washington-area firm about a job.