Yes, I've made countless purchases from US sellers on ebay over the last decade or so.
Paypal don't add anything extra (the seller covers the paypal fees) but the conversion rate from zloty to USD isn't as good as their local kantor will offer. If they can be bothered, the best solution is to have a USD account with their bank and to pay dollars in to that account which they have bought at the local kantor and then use the card from that account to pay paypal in USD.
how does shipping generally work in terms of pricing?
Each listing will say how much for shipping to the USA.
However, you might want to suggest to them that they use a shipping agent instead of waiting for your trips to the USA. I've been using these guys for a couple of years and have found them to be both cheap and excellent (as well as being pretty good at things not being stopped by customs): polonezparcelservice.com
 I've also been using Amazon UK for a fair few years and have only good things to say about them now that they have sorted out their delivery service (i.e. they now make very minimal use of the thieves at Poczta Polska).