With most of the daily/weekly newspapers going obsolete or next to nothing what is the best way to market "wanted to buy" ads for people that might want to sell their unwanted collections?
eBay, but there are a lot of fees. Cointalk is free, so its not a bad place to sell. And Numista is also free, but it's mostly for trading. Numista is basically cointalk, but for selling. I believe there is forums as well. Anyways, I saw your WTS list of silver and proof sets. On numista, you can leave a comment on your coins that others can see, and say $1.50 per coin and you have "xxx" amount of them. But you have to have a payment method like paypal and you have to communicate it with the buyer. https://en.numista.com/
I want to buy and I was wondering where to place "wanted to buy" ads for local collections in my area.
Newspapers are still quite relevant and are great sources for local for sale or wanted ads. Contact your local paper(s) and take out an ad. A place to advertise locally for free would be Craigslist. Just beware of potential scams from both and that people with collections may be hesitant to outright deal with you or to have you look through what they have as they don't know you. If you have a nearby or semi-nearby coin store in your general vicinity I'd be personally content to go through collections or collection dumps there.
The only thing with local newspaper ads is you are going to get everyone with a 1964 Lincoln "penny" that is really old and rare contacting you.