eh, the thing is, most of the time, if you are selling, say, kingston ram, when you buy advertising, you are trying to exploit a disparity in information access. I buy lots of ram, right? I've used three grand worth of ram so far this month and have 1/3rd of that on order. I want to pay the lowest price I can for acceptable quality (usually defined as being made by one of several brands.) ram. I spend a fair amount of effort seeking out the lowest price on ram.
If you are trying to sell ram to me as a retailer, paying money to reach me is probably going to kill any margin you would get from my purchase... to sell me ram you need to be the lowest price of a particular set of brands that I can find. Getting yourself on google shopping would probably be cheaper, and it would be just as effective (maybe more effective... I assume that when I click thorough a paid add, I expect to pay a premium on the other end to pay for the cost of that click through.)
Now, if you are trying to sell ram to someone who, say, doesn't know that google shopping exists, then maybe you can charge more than google shopping and then pay money for an ad to reach that customer.
So, in general, less-savvy consumers who don't understand how to effectively price shop are far more valuable to advertisers. And reddit users usually don't fall into that 'less savvy' bucket.
Now, this is true for the majority of retailers... people who are trying to sell commodity products.
If, on the other hand, you are trying to build a brand, and you can get your add to somehow imply that you are supporting reddit, or you are part of the reddit community (and there is room for that... by buying advertising space, you are supporting reddit, even though you have selfish motives in doing so.) you can effectively leverage that to build your brand amongst those more savvy users (and more savvy users tend to have control of more resources, over time, simply because savvy users generally make better decisions.)
So yeah... if you are trying to sell ram to me as a retailer, ads are probably not going to help you. But if you are a ram manufacturer or a ram "brand" (like kingston) and you are trying to put yourself on my list of 'acceptable quality' ram vendors, then advertising on reddit could possibly help you.