i thinks you have to target more specific audience whose are interested in your product. not all persons.
That depends upon person to person nature. Some doesn't reply and ignore others which can be annoying sometimes .The only thing you can do is wait otherwise they may mark you as spammer .
If you have something to sell, just post the appropriate ad. Don't spam people and always react to negativity with something positive.. Usually, if you have something to sell and its a great product at an acceptable price, it will usually never need much upselling. Try honesty or a new product or lower your prices. You won't make money everyday.. I never have online. But I still make a lot of sales, sometimes several in one day and nothing for 2 days.. Don't let one day of no sales ruin your day.
Life is busy, and when you are on the internet looking/buying stuff here on DP, one may be multitasking as is. I do note that there is a time limit between posts and PMs. Perhaps that's why. Folks read the Pm, nope that's not for me, next. Rather than posting a reply. Just a thought.
Online, it's a jungle - you have all kinds of people. You have friendly ones and you have those who are openly rude. You have scrupulous and you have scammers. Politeness and courtesy is a virtue that many people do not possess. Having said that, for someone who receives several mails pitching products and articles, it would be really difficult to reply to each person. Also, as someone pointed out, it's easier to let the offer expire than going ahead and refusing only to be bombarded with more fervent pitches.