transievers for cisco

When i look for 2500 series cisco routers on ebay some of them have transievers and some do not. When is it necessary to use a transiever?

It appears that you need tranievers to change the AUI type ethernet into an rj45 Type ethernet.

That's right. The old AUI interface (Attachment Unit Interface) allowed you to plug in whatever transciever device you needed for the physical medium you were connecting to. So if you want an RJ45 copper connection get hold of an RJ45 to AUI transciever unit and plug it in. They're simple and cheap; you should be able to get one off Ebay for next to nothing and some catalogues and vendors still sell them. These days however AUI is largely defunct and you'll only find it on legacy kit

so do you think using a 2514 is a good option to practice setting up internet and lan to lan routing ? Or is it to legacy to practice and study for ccna

No reason why you shouldn't use it for both