how can i secure tx?

[DigiSoul]

Or is there a way to verify that the person with whom i am making a transaction is trusted and trustworthy??

[NeuroticFish]

I think that you can do it with 2 of 3 Multisig address.
You will still need a 3rd, trustworthy person, but that person will have only to provide his public key and if anything goes wrong, that person can do the arbitrage.

I've seen on reddit some sort of tutorial for this, but it's for DOGECOIN.
The same principle works for Bitcoin too, but you have to study it more.

Of course, you still need a 3rd person. It's not really escrow, since the Bitcoin doesn't arrive to his wallet, but you need him in case of troubles.
Verifying a person you don't know is difficult. You can somehow verify a bitcointalk account based on the trust feedback it has, but basically that's it. Unless you want to hire private investigators, of course

[GregH37]

Suppose i want to make a transaction with an unknown person and i want to buy bitcoins from him .. How could i be assured that if i be sending him the money he will be giving me back bitcoins in exchange and what if he doesn't pay?? How to make this transaction WITHOUT USING ESCROW SERVICE?

Why do you want to do without escrow man ? The only possible thing that you can do is chargeback the money you sent to him if he does not pay but since you can do this hardly anyone would agree to sell bitcoins for such processors, because even if he pays you can charge back the money.
I will firmly suggest a escrow rather than playing with payment processors and ending up someone being scammed by other.