Economy shipping means two things, usually.
For packages 13oz and smaller, it usually means USPS First Class shipping. It's about $2-3, depending on the weight in ounces. It includes a tracking number, and that's about it.
For larger items, it usually means that it will go out via the cheapest ground delivery service the seller can find. I'd caution against using SmartPost without disclosing it in the listing- it's pretty slow. I usually use FedEx Home Delivery/Ground shipping for this, as it's usually cheaper for me than Parcel Post.
Very rarely will an eBay seller ship by using some stamps, and those are almost entirely cases of small, cheap items like decals. USPS First Class is what makes E-Commerce what it is.
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