I've bought a few and it's always a gamble. Shipping by far seems to be the big issue - shippers don't seem to understand how HEAVY a sewing machine can be (especially a vintage one) and they are often damaged in shipping. Sometimes the machine makes it OK but the case gets smashed to bits. Sometimes everything is wrecked. And sometimes everything is perfect.
I recommend reaching out to the seller and asking them how they will ship your machine. If it's a vintage machine that comes in a wooden case, I recommend that you ask them to remove the machine from the case and ship it separately, and yes that will cost you more. Otherwise, expect it to arrive broken - those wooden cases are fine for carrying your machine around but they are not designed to withstand the kind of banging around a shipping box often endures. Those heavy machines will come loose in transit and break the case from the inside out.
On the Victorian Sweatshop forum, there is a pinned thread dedicated to "how to ship a sewing machine" - if you're buying a vintage machine I HIGHLY recommend reading that post, and forwarding it to your seller. Be prepared to pay more in shipping, though.
When I'm looking at machines on eBay, if the shipping is anything less than $30 I usually give it a pass unless it's a machine I just want for parts. That price tells me they have NO idea what it's going to take to get that machine to me safely and it's probably not worth the hassle. (I buy heavy vintage machines, proper packing & shipping is going to cost at least $40. If you're buying a modern mostly-plastic machine, shipping will be less and probably doesn't need the intense packing a cast iron machine requires.)