There is nothing wrong with starting small and seeing what indeed will sell in your area or where you create a market. The problem with online and products is you are creating a job of packing crap in boxes and getting them out to the shipper. Usually about a $4 per hour job. Ebay doesn't care, they just want their 50 cents a sale or what ever.
If you really sit there and watch the products on ebay selling or rather NOT selling you get a handle on reality. However, this opens up the door of actually buyig something with the source being ebay bought low off season and sold for retail at Halloween time.
Another avenue it to just search the topic "Halloween Affiliate" and sign up for affiliate programs that will pay you like 4% every time some existing inventory sells something because they found out about it from you online.
Then you create some reason for people to come to your site like pictures of your project or product reviews or horror movie reviews and some small percentage of viewers might click on a costumes or masks affiliate and you get credit if they buy.
Yet, the actual amount of on line traffic is very low. It isn't going to be more than $6 a month even if you do well. Most programs won't cut a check unless you have at least $50 or $100 coming your way.
As far as buying crap at Transworld, many will be expecting a tax number and require a $350 minimum purchase.
When reviewing these affiliate programs, really look at the products and whether they are in stock or not. Crap they don't actually have isn't worth your time.
In the bigger picture, artisans who are selling at a convention certainly don't want to stock stuff and there is a given supply and demand opportunity there. You can send money to the vendor and get one in a year or you happen to have one in stock right now today for less because you got it at show prices.
The big failing I see in these "opportunities" is someone expects to sell last years big prop for $17,000 and won't go any lower or it is crap they want $40 that within days is at Walmart all of a sudden for $19.99.
The bottom line is, your rent will come due a whole lot faster than these products will sell or provide a commission. Don't quit your day job. It might not even make enough to buy a computer you will wear out and pay the internet service bill. In short you have been turned into a slave. It is generally fine if you are sitting on Mommy's couch, using Mommy's computer and not having to pay rent, the phone bill, buy something to eat etc. Only then it it free money.
Hence, on here we have events, buy and make crap and get the money from selling tickets. If you are going to work all year long to only make money once a year, it might as well be something you can somewhat control. Even if your event only puts $400 in your pocket it is probably as much or more than 100's of hours playing the rubber mask for lunch money market.
If you can't have your own event, go work with someone who can and pays. Sure the first event might give you $400 but 10 years down the line you have built the event to $40,000 in your pocket. Meanwhile the internet is still paying only $6 a month for 100's of hours. You have built nothing but everyone being tired of seeing spam here and there.
Another fabulous post from the U.S.Department of Wild Imaginings, now in spectaclar stereo, sponsored by the Adhesives and Sealants Council, suggesting ways to stick things together since the 1800s. Not fabulous in a gay way. Your results may vary. Illinois residents add 8% sales tax. These posts have been made by professional post makers, do not try this type of posting on your own without extensive training, lovely assistants and a trusty clown horn.