Originally posted by: princess ida
If you're talking about ebay, it's against their terms to add on paypal fees (or ebay fees either). Otherwise, just be up-front about it from the very beginning of the transaction. It's not cool to wait until after an agreement is made to spring something like this.
Yup, technically you're not supposed to pass auction or PayPal fees onto the buyer, but I say fuck eBay (the only reason they have this rule is to keep prices lower and entice more buyers to the site, eBay loves to screw over sellers and kiss buyers asses). I usually tend to factor some of it into the buy-it-now or shipping price (I'd assume a lot of people do this, even if it's subconscious). On expensive items, though, PayPal fees can really add up so after a certain point you have to just suck it up and eat the fees.
edit: These guys are right about adding on the fees after the fact, though, don't try to do anything like that. Now you know better, just keep these fees in mind next time you list an item on eBay. Or use Craigslist instead.