No mannequin is fine - just put the clothes on a hanger against a blank wall. The main thing is to make sure your photos are not blurry, colors are accurate, and you have photographed the item from all angles (front, back, close up shots of the tag, and pictures of any damage). Also, it is very helpful to list actual measurements of the clothes (sleeve length, length, pit to pit width etc.) since sizes vary so much between brands. Ebay buyers don't expect beautiful marketing. They just want to know what they're getting.
If you are selling a lot of clothes, it might be easier to list them all as auctions ending the same day, so that you only have to go to the post office once or twice. If you have everything listed as buy it now, you could potentially be making a lot of trips to mail things.
Envelopes are fine to mail clothes, but I think tyvek envelopes are nicer than manila ones. USPS has flat rate tyvek envelopes available (usually not in post offices, but you can order them for free on their website), or you can buy blank tyvek envelopes at an office supply store and calculate shipping for each item based on weight.