Credit card machines
These days there are lots of options for credit card machines.
Traditional telephone line machines like the Tranz 330 are extremely inexpensive and reliable. Printers for them are also cheap and use less expensive, non-thermal paper. They can process large volumes of sales. But the phone lines they require can be very expensive in many convention facilities, and some merchant service providers are phasing them out.
Cellular telephone machines can be acquired on the used market for about $400. These generally have integrated printers which take more expensive thermal paper, and some providers charge an access fee. They also almost always have integrated batteries, which is handy for low volume applications like promotional parties. Testing can be done ahead of time and there is no reliance on expensive facility infrastructure. They handle high volume well. Older machines can be a risk if they operate on decommissioned networks, which may not work in the facility or which may cease to work in the middle of an event!
IP machines are physically similar to cellular machines and in fact many cellular machines will operate in IP mode. This requires a reliable Internet infrastructure, which may be expensive in some facilities, but avoids problems with weak cellular signal in some buildings and can be cheaper to purchase and operate than cellular.
Paypal, Square, Quicken, and similar vendors have smartphone-based payment services with very low (typically 0) startup and monthly costs, at the expense of requiring a smartphone or tablet (or iPod touch) with Internet access. Printed receipts are typically unavailable. Percentage charges are typically slightly worse than with dedicated boxes, and these are unsuitable for high volume, but the low fixed cost makes them a good backup.