Lesotho's currency, the Loti LSL plural Maloti, is fixed at a 1:1 ratio with the South African Rand ZAR. South African currency is accepted everywhere — there is no need to change money. However you will get Maloti in change unless you ask which is very difficult to unload in South Africa.
There are ATMs at banks in most towns, although you will not find them elsewhere. Most banks will change travellers cheques for you, but it can be a very, very lengthy process if they are in any other currency apart from ZAR. Credit cards will be accepted in Shoprite and the main hotels, but not elsewhere. Your cashcard from home may work in some Maseru cash machines FNB or Standard Bank but best to get cash out in South Africa beforehand.
Restaurants outside of Maseru and most in Maseru will probably not accept credit card as a means of payment.
There are several Western style supermarkets in Maseru, which are good for stocking up on supplies in before heading elsewhere in the country.
If you're after locally made goods and crafts, your best bet is to give Maseru a miss, and head to TY or Hlotse, where the markets are far better and cheaper. You can buy traditional Basotho hats, sticks, rugs and various other curios.