Have you thought about re-evaluating which product verticals you go after first, based on cost of Adwords (or similar PPC) clicks? Sounds like your site/search solution can be targeted to any product bought over the web, so maybe you should re-consider which product searches to focus on as a matrix between the overall attractiveness and the marketing cost to get your initial users. I haven't looked at the cost of PPC for different verticals in a long time, but I'm pretty sure electronics is a very competitive vertical.