Dropshipping is the supply model of choice for most getting into e-commerce these days and of course with a limited budget, that's completely understandable. But of course, it's not always an easy way into the market, as I highlighted. Has it's advantages for sure, but as with everything, also has its disadvantages.
I couldn't really recommend one single thing to focus on for second hand, but the secret to buying second hand is to know your market. Pretty much everything is salable, but you have to tick the right boxes and they will usually be condition of the item and price. So I always suggest maybe focusing on products you know well, maybe hobby related and so on. Keeping an eye out for things you know is a good place to spot a bargain because you already know the market.
That aside, I would never go past buying something like old vinyl records at the right price personally.
Of course if you find things that you think might be a good buy, punch up eBay and check out some prices to compare and so on. Doing that is a great way to get a good idea on value of what you might get for an item.
The places to find the best deals, garage sales are my NUMBER ONE tip. The more upmarket areas are the ones to target, you can be surprised at just how much an item that some people consider worthless, can be valued by another and garage sales in great areas can net some absolute steals. That said, the deals certainly aren't limited to those kind of suburbs, you can find great deals at any garage sale, you never really know until you get there weather it was worth the trip or not.
Weekend markets of course, charity shops, even keeping your eyes out in the paper classifieds or even sites like eBay and so on. I've bought plenty of eBay and resold for good profit and week or two later on the same platform. Once you know the value of a product, then you know how to spot a bargain and take advantage it.
Another tip, don't be afraid to talk a good deal, especially a bundle deal. If you find 3 or 4 things that you want from a single seller, don't buy them individually. Ask what they want for each individually initially, then nego a deal on all 4 together. That's how you will always get the best price on the deal.
I don't have the time to do any hunting these days, but when I was doing a lot of second hand selling on eBay I worked on a 4 to 1 ratio. For every 50 I spent on product, I aimed at getting 200 back at sale. I did it for about 5 years, made power seller inside 6 months just starting out and I would say that 4 to 1 ratio was pretty close to just a base rate, did a lot better than that on most items.
So never underestimate the second hand market would be my advice. No shortage of products out there, you just have to learn your market, talk a good deal and get out and about and you can turn a very little bit of cash into quite a lot over time.