This is the first mistake which anyone can make is to deal with a person outside the forum. Even if the conversation on telegram is much faster, quick and in real time but you should first deal on PM on this forum and get the telegram name from that conversation and then continue conversation on these platforms.
This is the best and the right way to do transaction pm first here so you can check the reputation of the seller and you have proof in case something goes wrong on the transaction, on telegram people can imitate any legit guy and they can delete the conversation right away but getting an escrow is the safest way to do transaction especially if it involves a big sum of money.