I've bought quite a few things on Ebay, many brilliantly cheap.
I always investigate the market price first, and I always add in the cost of postage, including a Hornby Class 8 shunter with sound for just over £100.
I've also bought items that I have had to send back, mostly locos, because they don't work. Always got my money back.
But many people go crazy on Ebay. I just walk away.