A sales tax nexus is established when a business has a significant connection to a state, requiring it to collect and remit sales tax on transactions. Historically limited to physical presence (offices, warehouses, staff), the landmark 2018 Supreme Court ruling (*South Dakota v. Wayfair*) created **Economic Nexus** standards. Today, crossing specific revenue thresholds (e.g., $100,000 in sales) or transaction volumes (e.g., 200 orders) in a state legally obligates remote retailers to register and pay tax.

While US Sales Tax is a single-stage consumption tax levied only at the final point of sale to the consumer, GST (Goods and Services Tax) and VAT (Value Added Tax) are multi-stage destination taxes. In GST/VAT frameworks, tax is collected at each step of the manufacturing and distribution chain, with intermediate businesses receiving **Input Tax Credits (ITC)** for taxes paid on business purchases. This prevents cascading double taxation.

Although markup and margin are terms used to describe profitability, they measure different ratios. **Markup** is the percentage added to the net cost price to determine the sales price: `Markup = ((Sales Price - Cost) / Cost) * 100`. **Profit Margin** measures the percentage of profit relative to the selling price: `Margin = ((Sales Price - Cost) / Sales Price) * 100`. A 15% markup yields roughly a 13.04% gross margin.