First, thanks for setting up this forum & giving out some interesting facts about the forum's usage.
On the issue of trust, there really isn't any reason to trust an unsolicited email. As it costs almost nothing to produce, the sender is likely sending it to as many people as possible, which also means it likely doesn't have any value for you. Additionally, if their products were so successful, they likely would have chosen a better method of marketing to you. This is especially ironic given they are trying to sell you their marketing services, and you already are not impressed by their ability to market their own product. In short, I don't trust these vendors, and subsequently have no experience with their products.
As far as who do I turn to, this really depends on your product. The first suggestion is that you imply this process starts when it is time to market your indie game. I think this is indicative of the biggest mistake I've made, and the biggest mistake I've seen others make. The time to start marketing your indie game is the time you start making your indie game. Start off small and cheap (blog posts, facebook posts, twitter posts, industry talks, etc). When it is time to launch your game, you can then pursue paid advertising options, but these options vary wildly depending on your audience, your purchase price (or lifetime customer value), and your platform. Paid advertising and paid user acquisition can be extremely difficult to compete on for indie games, as even the medium sized app publishers are now spending six figures on marketing/UA budgets.
