Obviously, you are going to need a way to earn some extra cash to buy preps. One of the things that is easiest to do is buying and reselling used paperback books to second-hand bookstores. Those booksellers have a constant need for new inventory to replace what they are selling. They will usually pay 20% of cover price for books they want.
The best places to acquire these are garage and yard sales. Also, in some community type thrift shops. Forget Goodwill and the other "name" brand thrift shops. Their PB books are priced so high, now ($2.99 for a standard PB novel) that they are not a good source for this.
What type of books to look for?? For starters, mystery and adventure paperbacks published within the last 5 years. (Don't buy anything of this type, for resale, older than that.) Bypass romance novels. Bookstores don't want those. They date, too quickly. And they take up too much shelf space that could be used for other, more sellable, categories.
As a general rule, don't buy hardback books, either. Bookstores want the paperbacks, because those are the steady sellers. And don't buy beat-up or heavily used books. Condition is important. The store won't take a book with a broken spine, or one with loose pages that are falling out, etc.
When you have accumulated one or two boxes of PB books, look in your phone book under "Used Bookstores". Phone them to ask if they buy books in good condition they can use.....and what their buying hours are. (You don't have to ask what they pay. 20% of cover price is standard, and they all use that same ratio for buying.) Most bookstores pay in cash. A few will want to pay by check.
When you have found out who will buy......then just make the rounds of those stores. You will quickly learn who the best buyers are. For any used bookstore you go to.......be sure to ask what type of books they are looking for and would be likely to buy. The owners will be glad to tell you what those are.
You will learn more, as you get into this. It's fun.....and generates some cash income......and you don't even have to be a reader to do it.
There are other ways to supplement a SS check. But, this is one of the easiest to do, because you don't need a lot of cash to get started. Books at garage sales are usually priced at 10 - 25 cents each. PB cover price is $7.99. 20% of that is $2.00, which is a nice profit margin. Individually, of course, not worth the gas to go to the used bookstore. But, you wait until you have accumulated a box full of books to make that resale trip.
Hope this info helps........