Welcome to the site! Asking for help here is definitely a good idea. There are loads of really helpful folk around, and they know what theyre talking about too, believe you me. (apart from me. I talk outta my arse quite a lot

)
So - your questions:
GIMP is decent free software for editing/stitching photos together into montages (<--is that even a word?). As for improving the look of your mini, the first thing I would recommend is stopping the light source from shining directly onto it - it washes out the colour and projects a strong shadow onto the backdrop. Get a couple of pieces of paper and reflect the light that way (i.e. light source-->paper(reflect)-->miniature), or get a cheap white T-shirt and spread it over your light source (be careful with heat! don't set it on fire! If you can, build a wee frame and stretch the white material over it, then shine the light through that. Or, simplest option: buy a light booth. They aint too pricey.)
Anyway - some soft, indirect light will help a lot. Set your camera to a slower shutter speed (this reduces the amount of light you need), and use a tripod (or at least a pile of books

). It's also worth printing off a nice mottled/cloudy backdrop in an appropriately dark/neutral shade. I did this for the first time last week and it helped a lot. A lesson I learned the hard way is: in your photo software, bump the contrast up a bit. It's v hard to get an exact match for how the mini really looks and how it looks in a pic, and higher contrast helps things stand out. Again, I discovered this last week. (er...everybody told me so. I hadnt noticed, myself...)
As for ebay prices: sorry, cant help you there. I'm sure others can advise, though
So, good luck in your endeavours, and I hope you make a big chunky sale on your dark elf dragon chappy
