Ok, so I'm not all to familiar with tebex. But I do know that you can link your webstore to your server and (i think) make your own items and...
Ok, so how will you know the player purchased it? And what is the link you are using for them to purchase it?
I'm smarter than u because I have more chromosomes :)
I dont think that is possible sadly, but I have heard of these redstone contraptions called chunk loaders. You could probably try that!
So when you mean custom items, do you mean built in minecraft items (like a log, golden apple, etc) with different names?
Separate names with a comma.