Now thats the question...
Buying a sim card online can be a bit of a bonus given that most highstreet shops and even ebay sellers still have the nerve to charge stupid amounts of money for these cards when in truth you can actually get them for FREE!!!
I'm not kidding when I tell you that you can get them for free and that people who sell them on ebay have probably gotten them from the very same places!
O2 will actually GIVE you a number of sim cards if you ask them... You ring them up, give them your details and a few days later you get your cards, usually in sequence too which is handy if you want to give them to friends or family so you can easily remember their numbers. (You can claim upto 4 cards).
Things to be wary about when buying a sim, especially O2 genie sim cards is their expiry date.
All sims have one and invariably some sims will be close to or past their expiry date, especially if someones had a stockroom full of them for any length of time, though in my experience most sims tend to be fine.
This doesn't mean that its no good... it just means you have the added hassle of having to ring O2 customer services for them to reconnect you which can take a few days, not good if you want to use it right away.
Also when buying a "Genie" sim card you should note whether its an old Wap enabled sim or the newer GPRS enabled sim which gives you download credit as opposed to the old Wap ones which give you minutes for you to surf the net.
Again, this is not a problem because you can always change over to the newer GPRS allowance (1mb of download time instead of 300 minutes) by calling O2 and asking for it to be changed. This can be excellent for surfing ebay on your phone because you don't waste your minutes looking at lots of pages because downloading them on your phone is miniscule by comparison therefore you get more for your money.
The benefits of having a "genie" card (now called O2 online tariffs by the way) are excellent.
Top up £10 a month and you get 300 free texts... you can even change this to talktime if you prefer not to text or are missing those vital thimbs!!!
Top up £15 a month and you get 500 free texts which again can be converted into talktime instead and £30 a month gives you 1000 free texts a month or changed to talktime if you want to.
The things they dont tell you are the FREE stuff you can get by having a genie card... such as phones... yes PHONES!!!
By topping up each month you earn a bonus which is calculated over a year and if you have a sub-standard phone and fancy a more modern one, there is every opportunity for you to qualify for a brand new one from O2 or if you don't fancy a new phone you can get hundreds of free texts and talktime minutes.
If you top up enough over the course of a year you could get the latest phone absolutely free of charge and before you think thats probably 1000's of £££s its not... if you top up about £10 a week like lots of people do by the end of the year you could earn enough bonus points to get you either a massive discount on the most expensive phone they offer or even get it for free.
All these things are worth considering when buying your O2 sim card.
By all means buy them from the sellers on ebay. Just be wary of high postage costs and the fact you can get them for nothing if you are prepared to do a bit of homework to get them.
I hope you find this guide useful.