How do I contact many people interested in a specific niche, without spamming?

I tried a method suggested here which involves selling websites, and increasing the chances of a sale by contacting as many people as possible who are interested in that niche.

The site has sold, BUT the huge problem is that trying to contact people interested in the niche the website is about (by emailing people who's email address is on sites about that niche), got my Flippa website sale suspended until I agreed not to do that again!

Having tried to sell a website without contacting interested people directly, and getting zero bids, I can see that it really helps if I could contact people interested in the niche, but apparently emailing people who put their email address on a website about a certain niche, and telling them that there is a website for sale on that niche, is considered "spamming", and same goes for trying to get a post on any moderated forum about that niche subject.

So, are there any ways of contacting people who are interested in a specific niche, without it being considered "spamming"??? The method is for selling a new site, using a 3-day sale, so the timing would have to be short and specific, and the profit is maybe around $100, so it would have to be a cost-effective method.

How much would it typically cost to put an ad on one of the best websites about a particular niche, for 3 days?

any other ideas ?

Chris-