Truman grew up on a farm, and spent a few years on the family farm as an adult before enlisting in the Army during World War I, but he couldn't really be called a farmer. He dabbled at a number of jobs, and is usually referred to as a "haberdasher" - a men's clothing salesman. He also got elected as a judge even though he had no law degree, then to the US Senate. Basically, he was a political hack - a tool of the Pendergast Democratic machine of Kansas City.
Wallace was popular among the grassroots of the party but not among the party leadership, who thought he was too far left - some called him a communist. They wanted him out, because most of them realized that Roosevelt was very ill and not likely to serve out his full term is he was re-elected in 1944. Truman was a useful tool to that end, and was believed to be someone who could be controlled if Roosevelt died. Whether he wanted the job or not I don't know. From what I understand he didn't openly campaign for it but he also didn't discourage the efforts to put him on the ticket.