How do I turn a one-time sale into a lasting relationship?

You keep the accounts running, but you sense there is a deeper thing you have not yet learned, the shift from being a vendor to being trusted. That instinct is right, and wanting it means you are already halfway there, because relationships are built by people who actually care to build them. A transaction ends when the deal closes. A relationship deepens when the client feels you are on their side even when nothing is being sold. The difference is not technique; it is genuine interest in their success, shown consistently over time until they believe it. Start with one account. Reach out once with no agenda, no pitch, just to ask how things are going and to be useful if you can. Do that a few times and something changes. Trust is built in the moments you are not selling, and those moments are yours to give.