The vast majority of models you will collect are purchased on ebay. That has become the defacto marketplace for diecast collectible models. There are dedicated diecast collectible auction sites such as diecast.org but the activity there is nothing like what you get with ebay.

There are a number of things to consider when purchasing a diecast collectible car on line.

• It's easy to get carried away and want everything that you see. If your collection has a theme that limits the scope of what your collection should represent and narrows down the number of choices you have to make,
• Note that resin models are of a hardshell design. That is they have no opening parts. That makes it more practical for manufacturers to make the lower numbers of more obscure cars. The molds for resin models are also softer meaning they wear out after fewer uses. If opening doors, hoods, etc. are important to you then look for diecast metal models, Resin, however, seems to be the future as it allows a more precise scaling of the car without gaps for opening parts. Unfortunately you don't get a view of the engine or any underside detail and it's hard to see the interior inside a hardtop.
• The vast majority of sellers are honest and reliable. Use the feedback score as a guide but ebay now limits negative feedback to the last 12 months so it's not a definitive indicator.
• Sellers may change names or do business under more than one name so be aware that may be an indicator of some hidden issues.
• Having found reliable sellers give them repeat business.
• Know that for a particular identical item the price asked for 'buy it now' transactions vary greatly from seller to seller. Be diligent in your search for the best product price and total price with shipping.
• Free shipping may sound nice but if your package is sent slow, economy, last priority then the long wait may not be worth it.
• If a seller has enabled the option to make an offer then do so with a non-insulting, reasonable, number and most likely it will be accepted or a money saving counteroffer will be made.
• Look at the image of the car in the ad, If it is a stock photo and not a picture of the actual model being sold then the seller may not be in actual possession of the item. In this case you will have to wait longer or much longer for them to order and receive the model from their supplier before it is shipped out to you.
• Watch for cars not being represented as being from the actual brand or manufacturer that made them. The price may also be much higher then what it should be as the model may be represented as something it is not. I fell for this early on and only realized it when I saw the actual manufacturer's name had been scratched out on the underside of the model. As you become familiar with the various brands and their sometimes unique appearance characteristics you will be less susceptible to this deception.
• There are, of course, outright scammers and fraudsters selling products they don't have and will never deliver. Over time you come to realize a possible problem when nothing is shipped, the seller doesn't repond to communications, and the last estimated arrival date has passed. Only then can you open a case with ebay to get a refund. Scammers know all this and provide very distant estimated arrival dates. Be wary when an item has very distant estimated delivery dates.
• If you pay for your purchases with a credit card you have an additional layer of buyer protection. You can open a dispute with your card provider in advance of ebay's buyer protection initiation date. In my one case where this happened I reported a scammer who eventually agreed to issue a refund after ebay stepped in. Note they didn't actually issue the refund, just agreed to do it without actually doing it. Fortunately I had also disputed the charge with my credit card company who did credit my account for the amount of the charge.