1. Make sure you have plenty for sale
Nobody can buy something if it isn't for sale, right? Make sure you have lots of stock ready to roll. Scarcity is an artificial concept in this global economy, right? If anyone is suspicious as to how you just happened to have 30 Games Day minis to sell, concoct a tale of getting them surplus from a GW store that closed or something.2. Don't worry about cleaning the original model
Mold lines and vents are just part of the original sculpt, and you want to reproduce the original sculpt as faithfully as possible, right? Leave the original in exactly the same condition you acquired it in to preserve the integrity. Smash vents as flat as possible to the surface of the model rather than trimming them off before making your original mold.3. Don't worry if the recast isn't as good as the original
Most people buying these probably haven't even seen the original, so how are they going to tell? For all they know the original sculpt had that deformed hand or was missing that spike. Most people just paint detail on anyway.4. A little "used" look goes a long way
Make sure to soak the finished recast in simple green for a few hours to tarnish the surface, or put a three color paint job on the model so thick that details are obscured. If it looks like it was painted up many years ago or just sat around tarnishing in a box people are more likely to accept that it's an original model.5. If you take a photo make it a bad one
For your eBay auction, make sure you take the worst possible photo that still shows what the mini is. Using a web cam or an old camera phone with a flash are the best ways to do this. Alternately, include no photo of your own but just use a photo from the manufacturer's web site.6. When confronted, feign ignorance
"I didn't know. I got that in a trade a few years back from some guy at a bit swap," you can claim. Most buyers will just bite the bullet and accept that you sold on good faith.7. Enjoy life to its fullest
...because there's a special hell for people like you. Special hell.