I primarily sell on BST. Generally speaking SU sets have usually gone for 40-50% off the original price so I use that as a guideline -- except hard to find which are more, and common (like when people used to get a lot of extra SAB sets) for less.
Generally, for me, pricing depends on the condition of the stamps and also on the manufacturer and their popularity. Scanning through the listings can give you an idea, but sometimes it varies widely.
I like to post in my listing a request for the buyer's zip code. Then I package the item (typically a padded/bubble mailer) and weigh it on a postal scale, and then visit
www.usps.com and calculate the postage. I put in the exact weight... if I don't want to go to the p.o., I divide the postage amount by .44 and stamp it myself, rounding up. I figure that accounts for any slight difference between my postal scale weight and the p.o.'s and I'd rather have it go through with no trouble -- and the few extra pennies sometimes are just worth it rather than going to the p.o.
Once the postage is calculated, I PM the person back with my Paypal i.d. or ask for theirs to send a Paypal invoice. Be sure that they give you the shipping address if they use a different one in their Paypal account.
The p.o. will give you flat rate envelopes/boxes for larger orders -- and you can print out flat rate postage yourself also. For a weight, you just have to put "1 lb." because the weight is irrelevant on flat rate packaging.
Also, just a tip... on BST here, I like to change my individual items to a pale color, like Silver, when they have been purchased... I think it's better than deleting them b/c sometimes people will come back a second time and say "Gosh, I was sure I saw that item here!"
BST here has an added benefit -- I have met some of the nicest people in the world through there ... and have found lasting friendships, too.
Good luck!!!
Pricing & Shipping amounts?
