Hi there,
Lots of people have given you some really sound advice, I will just give my 2 cents also.
1. Do not try to sell in too many places, you will struggle to try and sell everywhere instead of selling only where it counts. Being based in the UK, the 3 biggest marketplaces are Amazon, eBay and Play. Play used to only be about selling their own media items (much like Amazon and books) and have recently branched out offering their own marketplace. They were also recently bought out by the Rakuten Group which means they have some serious money behind them and are definitely a place to look to get ahead of the curve.
2. Ignore Facebook, even the big players who put loads of money in to their Facebook stores have withdrawn as it wasn't worth it (notably GAP) and I read somewhere something I believe to be very true "Trying to sell to people on Facebook is like trying to sell to people in the pub whilst trying to socialise with their mates". That said, it can work for some people but in every experience I have had, it has tanked.
3. Get a webstore, check out how it is with SEO as some will be better than others. A good way to see this is to search for people who sell similar to you and see who is at the top and what they use as their webstore platform, it is usually at the bottom in the form of a link.
4. Get a eBay store design, costs a bit but the return on investment is very high, especially with clothes. Make sure you use high quality images and place lots of pictures within the listing so it doesn't cost any extra.
5. Make sure your images are SEO friendly on your webstore, put in effort here because especially with clothes people will go to Google images and search for what they like and will see images and when they click the images they will be taken to your site.
6. Use a multi-channel management system to help you list, manage listings and inventory, manage your stock, manage your orders, host additonal images for you and some are very good at creating variations *cough* ChannelGrabber *cough*
7. Once you have established yourself on eBay, Amazon and Play in the UK, start to branch out to Europe and the US. The US has the benefit of same language and same economy and fashion etc. however the postage takes longer and may be harder to manage from a customer service perspective. Pick European countries close to the UK first, with good postal services and with similar economy and fashion (France and Germany are good, as is Netherlands but Amazon NL is not out yet and eBay NL isn't the best for selling out there, they mostly use Marktplaats)
8. This ties in with webstore and point 7, when selling to different countries, people like to read in their language. You can even use Google translate to start with, it doesn't need to be perfect, heaven knows that English speaking British people who sell on eBay have appalling grammar most the time and it is the same in other countries, they don't expect perfect but they won't put up with English.
I could go on all day but would not like to send you to sleep!
Daniel Williams
Managing Director
ChannelGrabber
Simplified (multi-channel) Selling