Well the conical orifice is what allows me to use special materials. But the size of the bobbin can have an impact too. Try to get a wheel that allows you to switch out to different kinds of bobbins.
I would also recommend a double treadle. I just think they're easier to use.
There are plenty of old fashioned wheels out there that cost less, but really there's no reason other than price to go without these features.
Yeah, I sold yarn on etsy for a bit when etsy was new. That was before they had fees and listings expired. I'd still post there if the listings never expired, but I don't like the new rules.