Hi ag,
I suggest to start out with a minimum of of small to medium devices (not physical size but functions), say 10, to offer either as ready-to-go circuit or as a kit.
I wouldn't use ebay to sell my products. Altough ebay takes care of proper business (payment and delivery) the amount of money you have to pay for the service is probably too high for the beginning.
For marketing I suggest use banner advertising, which you might get at low prices, and make up an own website offering your devices.
Don't forget to make your general business conditions public before you go to sale. That way you can cut off unqualified complaints. (Sorry, can't formulate better) It means exact terms of warranty and exemptions for it. Don't forget any major point. Otherwise you might be confronted with the claim to replace an inadvertantly destroyed device. (Anything not covered by conditions means unrestricted warranty)
Depending on your location you might also throw in newspaper advertising from time to time. (don't do newspaper advertising if your city's population is below 100,000. It's pure waste of money.)
From my personal experience I suggest to also offer different payment methods, including an internet payment system like PayPal or Click&Buy. For me (having no plastic money = Credit card) it is difficult to purchase certain things via the internet.
Kind regards and luck for establishment of your business.
Hans
edited for typos