Further to what the others have already said, if you feel it's going to be a long term venture, buy your envelopes in bulk. I bought 200 small padded bags for $25.50 with 'free' postage. The padded bags are 180mm x 100mm. If the item including the padded bag is under 20mm, it usually costs me $1.40 to post (my items are small and light). Anything larger than 20mm, I send as a parcel in a box. 

The seller I bought these from has all sizes of every type of envelope and when I was looking around for them, was the cheapest on eBay at the time (in Australia). Here is the item number for the ones I got 220678678187 and you can then search the sellers other items if those ones are too small for your needs. They run both buy it now's and auctions, but most of the time, the final auction price is higher than the buy it now price, so easier just to buy it now. I received them 2 days after payment.

For the first few sales, take them unstamped to the PO and make sure they come in under 20mm. Once they exceed 20mm, you get charged parcel rate, which is $7.20 for up to 500g. Once you''ve posted a few, you'll get to know whether they are the right size or not. If the items are difference sizes, make a 20mm slot that you can check the items before postage. I just measured one up on some thick cardboard and cut out a hole 20mm wide. If something looks borderline, I'll package it up, then slide it through the slot.

If they are just things like the clear plastic screen protectors, they could just got in a normal envelope with a 70c stamp (providing they are under 5mm).

If you plan to charge for postage on your items, make sure you factor in eBay's fees. They take 9% of the sale price and 9% of the postage cost, so factor that into both the item and the postage. If you will be offering free postage, remember to factor the postage cost and fees into the purchase price. If you will be running auctions, don't start them at 99c. Start them at the lowest price you are willing to accept for your items because often things sell with only one bid. The last thing you want is to be paying people to take your items off you. You need to make a profit, not just eBay.

Good luck with it all!