A currency exchange is a business that buys and sells currencies. Currency exchanges offices are dealers that mainly buy and sell currencies. These shops also offer almost always the possibility to buy or sell traveler's checks. You can usually find them on touristic locations or in airports. These traders/dealers are able to earn a living thanks to the difference between the rates at which they buy or sell currencies. Some of them also ask for a fixed commission.
You can order currency at your bank or through a money changer. This topic is addressed in depth in this article: Where to exchange money into foreign currencies ?
No, there is no threshold but the money changer may not have enough cash if the the amount is a large number!
A traveler check or traveller's check is a preprinted, fixed-amount cheque that is accepted in many banks and exchange offices worldwide. The advantage over carrying cash is that you are covered in case of loss or theft. More information here: All there is to know about Traveller checks
It is up to you to decide if the extra security provided by traveler's checks is well worth the cost and burden of using these checks.
Most money changers and currency exchange offices refuse checks ( not traveler checks though !). Credit card or debit card are sometimes not accepted, some offices prefer to handle cash directly.
Yes it is quite possible and even smart because you will benefit from more competitive rates than by going directly to a currency exchange. Besides you save time because you do not have to go out.
When it comes to money there are lots of scams. We invite you to discover our article discussing this : Travel money scams
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