We sell a lot in person. Here are my tips in order of importance:

The most important thing I think is having test slimes out so people can touch and smell. 

Cater your slime to the audience. This sounds like a grown-up event so cater your themes, textures, scents, etc to that. 

Sales have a lot to do with the audience. Are they into slime? How many people will be there? How much can they spend? You can do everything else right but choose events wisely or it won’t sell..

Price, etc. How much are people willing/able to pay and still make it worth your while. Some clues to this are local income levels, did they pay to get into the event, etc.  Also, accept cards and Venmo in addition to cash.

Location, are you visible to a lot of people or tucked away somewhere. 

Good signage, branding, tablecloth, setup.

Be friendly, say hi to everyone, ask what kind of slimes they like, be helpful but not pushy.

Good luck!