The item is worth what it will get at auction or a 'Buy Now' price.
I have made quite a number of computer purchases via eBay, but be warned, while their are bargains to be gained, there are also some badly described items that people might want to sell. Always check feedback results, and protection policies.
If at all possible, try to buy something within reasonable travel distance, so you can actually see the item working, if the price warrants it. Most genuine sellers will allow inspection before the final date, and these same people will usually respond to emails very quickly. If the don't, then do not purchase, no matter what you consider a bargain.
Another bit of possible advice depends on the age of the item. Laptop batteries can be very expensive with a limited lifespan. If its a registered business seller, check any warranty and returns disclaimers. Private sellers 'might be' exempt for claims?.