"Any old" first customer will not do. Start up customers should be purposefully sought out and sold to. Before you make your first sale define who your ideal customer is and go after them.
The excitement of a sale to a random customer is wonderful but it may lead to a fuzzy description of your target market. This weakens all of your marketing and sales efforts. When you know exactly who your start up customer is you can market more effectively to everybody else who fits the same customer profile. You'll be in a better position to leverage your start up sales into long-term, ongoing, repeat business.
Start up customers won't let you down if they fit the market your products and services best serve. Random customers will produce a low volume of repeat business. Targeted customers will typically build your sales volume with repeat business.
Start by purposefully marketing to a defined market place of start up customers. Don't let the market place of random sales define your start up customer list.