it ain't amazon but it ain't rocket science either, lets get started:
0 get bitcoins
there's a guide
here but lots more on the web.
1 pick a seller
go with a solid reputation. new (< 10 sales) sellers aren't bad but require vetting skills you don't have yet. note when the seller was last online but don't necessarily worry if it's been a while; send them a question and this will generate a notice that brings them online.
2 REGISTER
though you can buy anonymously its better to register on little biggy, since a made up username doesn't identify you anymore than no username it's not really disadvantageous. if anything goes wrong follow up is much easier and you get to leave feedback which keeps sellers in line. you also build up a reputation which leads to privileges.
2 BUY>PAY
once you've selected your item press BUY (duh).
enter your correct name and shipping address (deliveries can fail without this) and hit PAY. you will be transferred to a payment processor site where you get a bitcoin address to send your payment.
open another tab/app to access your bitcoin wallet/exchange and send the coins to the stated btc address. within a few minutes your payment is received and you are forwarded back to the little biggy "order page". if for some reason this you lost the tab don't worry, bitcoins have receipts built into them, just let your payment processor know what happened and you'll get sorted out.
3 order confirmation
if you've registered there's nothing to do here, within 24 hours your order status should change from "paid" to "shipped".
4 leave feedback
after your purchase you'll see a red number next to "ORDERS" return reminding you to go to that page to leave feedback, do so as this is what keeps things humming.
* problems
if your shipment hasn't arrived within 11 days hit the dispute button that now appears on your order page. again it's helpful to be a registered user since your buyer reputation is a factor (along with others like the seller's reputation, recent activities and the existence of shipment tracking).
if your problem is quality rather than shipment it is best to work it out with the seller. if you are a registered user you can leave feedback and that's important to your seller especially if there is a consistent problem happening being reflected by more than 1 user.
always keep biggy's positive vibe in mind. hijacking threads and rude posts don't earn respect here and are bound to cost you. if you need to vent always give people the benefit of the doubt and show that your words are sincere and unselfish, you'll quickly make friends and earn respect that helps you.
happy buying! if you need anymore help just use the topics threads on this page.
