I would recommend you have a focus in your collecting before rushing out and buying everything which catches your eye at the time. Many collectors have shelves or attics full of boxes of models they paid full retail for which eventually have little meaning to them. Decide what airlines, what Country, what military, what type of corporate aviation or from what time period interests you most. What scale do you like or have room for. Do you want modern or vintage or both. For example my focus is from my early days in aviation 1970's & 1990's- airline, military & corporate around the Calgary area. I prefer vintage models of all scales, but I do have some modern die-cast Inflight200 airliners for example. (highly recommended by the way) I have a couple of snap fit and several more vintage Air Jet Advance models. Someday I would like to have a house large enough to have that large vintage $12,0000 cutaway Air Canada L1011 model that used to be in the window of the old travel agency in DT Calgary I used to press my nose to the window every time I went by.
I have a wish list and then I go out looking for that specific model. The wish list is often updated when I discover something very cool and decide to add it to the collection. I have my collection on my own website. Here is my wish list. http://www.skythings.ca/buyselltradewan ... wish-list/
Sometimes it takes many years before you find it. It's all about the hunt and the challenge. Please note, none of the models on my website are currently for sale.
I use eBay to purchase many of my models. Kijiji is a great resource as are forums like this. When you purchase from the retail modeller sites, you most often pay full retail. The same model will typically sell on eBay or Kijiji for 25-50% of retail. Some have become very collectible and sell for double, triple or even 10 times the original retail value depending on the demand. Here are some other websites you can check out for yourself depending on your interest.
http://www.aircraftdisplaymodels.com/fo ... =179&e=179
http://www.aviationmodels-online.com/fo ... spage1.htm
http://www.collectspace.com/cgi-bin/for ... LastLogin=
I collect and restore Dinky Toys brand diecast airplanes, as well as others in that product range. Some date back to the 1930's. I also have many DHC models, as I made many for deHavilland back in the '80's.