Replace with second hand modem RIDICULOUS!!

We purchased a new 3G device after explaining to the saleslady Dominic that our one was faulty, and we did not want to upgrade, but just to use our existing sim card. She said this one would work and so we bought it.<br> That same day we went back in the afternoon to say our computer showed that no device was installed in our computer, the technician Tebogo tried it on his computer in store, and it worked, so they said we needed to go and retry it and install it in our computer.<br> The device installed on our computer but when it comes to using it to connect to internet etc it says NO DEVICE.<br> We returned on Wednesday 30th December 2015, with our computer to see if they could load it or sort it out. Cradlestone vodacom sent us to a technician at Key West, 3 hours later between the two stores. No one could assist us, so we requested a refund.<br> Only to be told they do not do that, we had to send the modem in to see if it was faulty, which we knew it was NOT, obviously our computer will not read the new modem. We were then told we will be given the store managers OLD modem with the same serial number as our old one at the price, this is USED not NEW and we insist on a refund. Please Assist.