I have been casually selling comics for about 3 years I would use mycomicshop.com. Loved them cause it was easy to just send them the slabs and I would throw some in weekly auction some in the big prime auctions they do 4 times a year and leave some as just buy it nows. I have switched to selling full time since July and have learned a little mix is pretty good. For books Over 500 I still use mycomicshop for the most part since I am paranoid of being scammed. Recently started testing the waters with little bit of higher value stuff on ebay. Usually books under 500 I list on ebay and local use FB Market place and app called offer up surprising good amount of local sales but I think that depends on your area I am in a large city in Texas. For bulk since I started buying collections I just pull keys or complete runs to sell my self. The rest of the filler I break up in to short boxes and sell them 30 to 150 per box depending what I put in them. Have themes to the box All Marvel, All DC, x-men, FF, Spider-man, Batman etc. I only sell bulk local cause the cost of shipping plus I am open to trades for bulk or deals when people buy more boxes. I don't have enough room to store all of it so I keep my inventory tight, keep the bulk flowing out quick as possible. The best way to get fair value would be to sell smaller books on ebay first to get your seller rating up then start with the bigger books just be prepared to get close to FMV is going to take forever. Some stuff sells in 15mins some stuff takes months! Its a slow process but once the ball is rolling it starts picking up steam. Also push for local cash deals if you can that is usually best but you meet all kinds of different people it really shows you how diverse the hobby is.
Mycomic shop fee is 10%, Ebay is 13% to 30% depending how much of adrate you run. Also don't forget about taxes on online sales. Local FB/Offer Up I only takes cash... for reasons... So no fees but I also give a discount for local cash deals.
I don't even mess with IG since I have never really used social media so to get a following will take to long now that I got my sea legs going to start posting more on social media but would not start with that. Who is going to want to buy a book from a dude with 3 followers... Also don't do FB sale groups cause I am poor and cant eat the loss if I am scammed. Be sure to check out the ebay blocked buyer list on the forums people add to it all the time super helpfully I just add who ever is on that list. When some one ask why they are blocked I just tell them some how you get on the bad side of some one on CGC forum and move on with my business. Wish you the best of luck with what ever you decide to do.