We get a lot of people come in with the "I can get it for £x on eBay....", but it's as you say, some things you need straight away, others you're not really sure of the quality on.
We used to make our own RJ45 (ethernet) cables, but unless you're doing it for a fixed installation (offices etc) we found that the molded typed (which we sell now) are better for home use as the wires tend not to come out, and if they do people don't try and refix them, then blame us when their hub doesn't work
eBay is cheaper, the vast majority of places on there don't have fixed premises, low overheads and minimal staff, it's like everything in life, you get what you pay for...