Daniel,
Macro is technically where the size of object being imaged and the size of the projected image on the CCD/film surface is at least 1:1
ie same size or larger.
This means that a cockroach for example might fill the frame.
"Macro" tends to be incorrectly used generically for magnified close up shots.
The ebay macro adapter lenses do work but generally they are of cheap construction to say compared to an chromatically corrected doublet like the Canon 500D macro converter or the Olympus MCON-35 (they are what I use)
Those ebay ones are probably uncoated or maybe single coated and just a single lens element.
But they will work but may give you problems.
There are various sizes (diameters) of these and other brands have equivalent adapters. Almost all of the different brands of macro adapter lenses will work on any brand of camera or lens, just make sure its not too small in diameter to fit or cause vignetting.
Whats the filter size of your camera lens.
Ideally you want to prevent internal reflections between your primary lens and the adapter lens - so multicoated doublets are much much better.
You probably won't use less than +3 diopters.
A macro lens is designed differently to an ordinary lens (plenty to read on the net about it) and these usually give the best results, but can be quite expensive. (my preference for use)
Some telephoto zooms just happen to have the right physical and optical properties to work really well for quasi macro and with macro adapters - they allow you to shoot butterflies and dragonflies and other skittish critters that don't allow you to get too close.
Since you are magnifying the object and introducing more glass you lose some light, but once you get down to macro magnifications you will find that in order to get any useful depth of field you will need to stop down - by as many stops as your setup will allow you before you hit the diffraction limit.
The depth of field wide open could be less than 0.5mm so a spiders eyes are in focus - everything else gets blurry.
Even at f22 it may not necessarily get that good that any more than half the spider is in focus.
Thats where macro stacking (same as astro) can be useful but in order to gain an increase in depth of field rather than noise reduction.
Here's an article
http://www.wrotniak.net/photo/e10/mcon-35.html
Once you stop down you lose more light and so stability becomes an issue - so get a tripod, focussing with macro is often better accomplished by moving the whole camera and lens assembly, many macro photographers use a small rack for this purpose. It sits on the tripod between the camera.
Also you may want a tiny little tripod for shooting objects at ground level
Just make sure that you keep an eye where the front lens element is - its quite easy to poke the lens into the subject with some lenses - thats how close you can get.
Flash - once you get into it, a flash becomes an essential tool - in fact the best setups have two flashes to reduce shadowing, or use a circular ring flash.
There are now cheaper LED ring lights available, these screw onto the front of the lens
Flash overcomes the problem of a stopped down lens and poor lighting and increases the speed at which you can tolerate your subject moving - like freezing the wings of an insect.
Suggestions - use a Flash - preferably a double flash - cheap flashes can be made to work quite well.
Oh and finally the use of extension tubes - these fit between the lens and camera on DSLRs (not sure what yours is) - some lenses can allow them some cannot so check first.
Hope that helps for starters
Rally