huh???

I thought it was easy to understand
If you have a goods or service that you would like to pass on for free, I may say I have a set of wheel chocks I no longer need, I would then put a post on the forum stating that I have a set of free wheel chocks.
Milo may see the post and needs a set of wheel chocks, he would then contact me and We would come to an arrangement for Milo to get the wheel chocks. Now Because milo accepted the wheel chocks in all fairness he would then decide on an item that he no longer needs and is prepared to give away. So now MIlo may be able to tune a musical instrument, so he then puts a post up for a free tune of an instrument. Now Jimricho may need a musical instument tuned so he contacts milo etc
You now have a pass it on/pay it foward.

I give to charity all the time and don't have anything I can Pay it Forward with UNLESS someone has a lovely campervan to give away and in return I will give someone else a yellow Hyundai Excel twin cam in good condition.
Any takers?

This is quite different to barter. With barter you exchange goods/value with others directly or a credit is registered in your name. Barter exchanges are legally taxable.

Pay it forward may never result in a return for the goods or services offered and this is not a requirement for it to work.
Accepting a gift applies an obligation to make a gift to someone, anyone.

Cheers,
Peter

There was a movie once with that title.
That is the idea. Someone gives you something they may not need anymore or just a generous gift. When you are able you do the same for someone else.
We all do it at different times in our life.