Reselling and dropshipping are two very similar types of business, but you may need to understand their differences so read following:
How reselling works?
You find a product which can be sold at a higher price, but as reseller you need to buy it first then sell it, to make a profit. So investment is required. Plus you have to be sure that your customers will be satisfied with that product. If your buyers for any reason want to return the product and get a refund, then you can be in loss if you can't return the same product to your supplier and get your own money back. In this case, you may lose money on transactions or even on shipping costs if it's a physical product. So you must be experienced and sure which products or services you buy for resseling and keep in mind this risks.
Is reselling legal?
If you have an agreement buy your supplier to resell their products or services, then it's absolutely legal, do not worry. Btw, any business would like to sell more, even if you adding your own profit on top of it, they still selling to you at their own price, or even discounted.
How dropshipping works?
Very simmilar to reselling. But for dropshipping you may need special agreement by product supplier. So dropshipping is when you find a customer for a product and sell it at your own price as you wish, then when it's sold, you passing details to your product's supplier and supplier send the product directly to your customer. So you are here just middleman. Almost no investment is required, because you will receive money from your buyer, keep your profit then buy the product for rest of the money you already received, plus you not need to provide or ship it your self, it's done by your supplier. How good is that? Well... Awesome In most cases, it's easier to work then reselling and it's much less investments and risk involved.
Hope this comparison helped you to understand this two types of business and it's differences.