A great way to fill the wholes in your collection without coughing up more money is to trade what you already have. We've all heard the expression that says that one man's trash is another's treasure. I learned this doing my first comic con last year. I had a bunch several doubles and multiples of issues that I bought in bulk. So instead of taking them back whith me I figure why not go to the other dealers/sellers and see if they would trade with me for some of their books. This really paid off as I was able to acquire a big chunk of books I wanted with some of my old new stock.
Now if you're in some of the FB groups you can try a few different strategies that have worked for me. The first is to create a post asking for references from the group for both yourself and the other person you're wanting to do the trade with. You'll get people that will comment to support that person. And at times you'll also get candid advice via messenger about the person that wanted to trade.
The 2nd method is to come to an agreement on what your going to trade with values. I recommend screenshotting the conversation so that you have proof if issues arise in the future. Then you take the value you're trading with each other and send to each other in that amount through Goods & Services in PayPal. This way you're both covered. And at the end when you have received your trade and both parties are happy you refund each other to balance out the transaction. So the PayPal transaction acts as an additional layer of insurance on your trade.
I advise you to make sure you ask any and all questions in order to make sure the book you're getting is going to be at your expectation. So asking about the condition but also ask the person how they ship books as well. You'll also want to figure out what type of shipping they'll use so that you can match that in order to keep everything fair.
Got several books you can't sell? Books that you no longer want? Use them to your advantage to find the right person that will trade you for what you want.
Good luck on your first trade!