The real answer is that no matter how much, or how little, you pay for that item you sell for a dollar, you are going to lose money if you only sell a single item. You must consider all of the other costs you must pay just to do business.
I will use the example that others have used. You pay $0.50 for an item that you sell for $1.00. You make $0.50 gross markup each. Not a bad markup in general.
If you have a brick and mortar store you pay rent and utilities. You pay employees. You have everyday business costs like advertising or marketing. If you have a dollar store and a customer walks in and buys one item for $1.00 you lose money. The cost to handle that transaction is just too high. Hopefully, your customer will buy 10 or 15 items at $1.00 each and allow you to make a little profit.
The same principle holds true for e-commerce stores. Your overhead may be lower but you still have to pay for someone to process the order, print the label, put the item in a box, and ship it. You need to pay shipping charges. You need customer service to notify the customer about tracking numbers, handle questions and returns. All this costs money.
Only you can determine how much it costs to process each order. Calculate how long it takes to process an order, print a label and pack and ship an item. Next calculate your labor rate and apply the cost of labor to the time spent to process an order. Add in credit card or Paypal processing fees, selling fees, advertising and marketing costs, and the allocation to overhead you charge for each order. All those business costs add up pretty fast. You will find there is not much left of the $0.50 “Profit” you think you may make.
I guess is you sell an item for $1.00 each, are completely automated in your order processing, and you sell thousands every month, you may be able to squeak out a little profit at the end of the month, but it will be really tough.
Good luck finding that item!