One of the most important aspects of eBay selling is getting the weight of the shipment correct and calculating the postal and packaging costs.
If you are serious about your eBay business then invest in an accurate set of digital scales. It is essential that you can weigh in grammes and kilogrammes, your average kitchen / bathroom scales will not do!
We obtained ours through eBay, where else?
The scales that we use are American, trading under the name of Ultra-Ship.
These have proved to be reliable and accurate.
Another useful measuring device is what I call "spring" scales, these are similar to the scales used by fishermen to weigh their catch, just a simple hook on which to hang an item and a rotary scale to give the weight.
This particular scale is very useful for odd shaped objects, we recently had need to weigh a guitar case, something that we couldn't fit on our conventional scales.
They are very useful also for weighing clothing and any other item that you can place a wardrobe hanger into.
Bear in mind that these will not be as accurate as a good set of digital scales so allowances must be made, best to weigh something that you know the weight of over a selection of weight ranges to calculate adjustments.
When weighing items be sure to include the weight of any packaging, generally the heavier or more delicate [i.e. electronic] the item then the more packaging. Packaging = weight when you get to the post office counter and if you haven't calculated for it you will lose money on post and packaging costs.
Hopefully if you follow these guidelines then the amount of money that you lose through mis-calculation of post and packaging costs will be minimal.
In general you will find that customers will appreciate careful packaging, so don't stint on the content.
On the other hand nobody will appreciate you charging higher packaging charges if the items arrive in a package that is insecurely packed.