An unsolicited telemarketing call is usually the first sign of a scam. Did your mom even have her TS listed for sale? Nobody legit calls a TS owner who doesn't have the TS listed, offering to buy.
I have never heard of them but likely they have already been in the TS resale scam business for a while but just under a different name. True, they might not have asked for any money yet, but here is what usually happens. The scammer will say that the sale is ready and your mom has to put some money into escrow which will (allegedly) be refunded to her on closing. Or they will say that there is some sort of fee or tax that has to be paid in order to close the deal.
Once they get the money from her (hopefully they don't), they will disappear and cease contact while making off with your mom's money. We see and hear of this scam weekly here on TUG.