I would recommend https://www.mycomicshop.com/. I buy from them regularly (20 to 30 books a month) and they are strict at grading, masters at packaging, and fast at shipping.
https://www.scottscomics.com is another. Just be aware that shipping is slow as it is a part time business, but the grading was spot on. The site has a limited selection of back issues, but you can probably find some gems if you willing to dig.
If your are looking to just fill a run fast and cheap then you can also try https://www.newkadia.com/, but just be aware that grading is not as strict as you will find elsewhere. I would stick to VF+ or higher if you want a decent book and be sure to pick books that are discounted and you can apply a coupon against. The best thing about this site is they list Overstreet prices at the top right of each book so you can see how the cost compares. I do advise that you price comparison them against https://www.mycomicshop.com as they might actually beat NewKadia at times if your buying a lot of books at once.
I have not used Instagram yet, but several of the YouTube channels I watch swear by the IG comic community. Just avoid the book raffles as they are everywhere.
I also hear that local comic/pop culture Facebook Groups can be a treasure trove for finding books.
As for the other non-eBay auction sites, be sure to read the details about buyer fees and shipping cost before you start bidding as it is not the same as eBay.