Hi atticblingandthings, and welcome to SaleHoo.
I love it when I hear someone is developing their own site, gives you far more flexibility than an already made platform. That being said it also has it's drawbacks, and traffic generation is always one of the biggest.
You could offer the best deals on the planet, but if no one knows about them, then you' aren't going to be selling much at all. Traffic comes down to marketing, and there is many options available as far as marketing goes, but the general rule of thumb is that good solid effective online marketing costs a decent amount of money.
My suggestion, go with both. Move forward with your own platform absolutely, but diversify your market by using sites like eBay as marketing tools for your own site.
Now in regards to advice on developing your own site, I could write for hours lol, but it's a subject that is probably best target specific. For example, ask a question in regards to any area of the development at all, and that will make it far easier to provide pointed advice.
Plenty of good business minds on the forum here at SaleHoo, from a range of diverse business backgrounds, so you're in the right place for advice for sure.
Now in regards to shipping, what country are you shipping from, what countries are you shipping to? Are we talking about single item sales or bulk shipping, and what payment options will you be accepting?
Looking forward to seeing more from you.