Game codes is not received to resellers

If you purchased a code to use in the xbox live store and downloaded the game you cannot resell it. If you bought a disc then you can resell it.

And what error message do you receive when trying to redeem the code?

You cannot redeem trial codes for game pass if you ever had game pass on your account before. Doesn't matter if it is from a promotion, trial period or purchased.

But if you never had game pass on your account in the past, you need to contact the seller of this code. Xbox is not responsible for unofficial third party resellers' codes.

You will have to find your own methods

As code reselling has conditions like region etc

And you will take a significant loss

There are lots of code reselling services but they take a cut

It may help if you explain what games, or codes you are expecting to receive, and also why are you expecting to receive them.

If you are referring to the Mixpot (which is the only thing recently that you may have been expecting a code for) the games are assigned directly to your account, but it usually takes a week or so to be done, if you qualified, however as there is very little
to go on it hard to be sure what you are asking about.

If you are referring to games bought by Microsoft they usually get applied directly to your account.

If you are expecting a bonus code for buying a game via Microsoft or Mixer then you may need to say what it refers to.

If you are expecting a code for something like a Quest reward, again please say specifically what you are expecting

If you are referring to codes from a specific reseller (such as bonus exclusive content) you need to contact that reseller.

Sorry this is vague but its hard to know what you were expecting to get.

Contact the reseller who sold you the code