This is no way to run a business, nor to sell a PC privately, on eBay.
You (the seller), don't send out PC's without any XP disk or installed OS, and with just a license sticker on it, and say, "You don't need XP, just get someone else's XP CD". That is plain ridiculous.
I would complain to the seller, and send a copy of the complaint to eBay's complaint department.
There are a number of issues with this seller which I would have walked away from.
1] These are listed as used Dell Optiplex GX260 Intel P4 1.8GHz 512MB 20GB XP Pro. The eBay page you posted shows whole pallets of these Dells, and in a [quoted] Q&A he says;
Q: how many are on pallet and how much is the pallett
A: Hi is 60 on pallet,and we can do 60.00 each. mirek 07769658999.
How is he getting whole pallets of Dell used machines? Do Dell sell them off in bulk like that? If so, do they sell with XP pre-installed? Do they sell with the XP licenses?
2] The photo's have the XP logo on them. Why isn't XP pre-installed?
3] This appears to be a UK business. His name and address is;
320 kings road
Foreign name, (puts me on guard, if not in itself suspicious), and look how he quotes his post code, rg1 4jg. Should be RG1 4JG. His address is kings road, not Kings Road.
When I am dealing with people online I want things to be 'perfect', not slapdash.
4] Google shows Mirek Limited as, amongst other things, a (Office) Cleaning Service, a photocopier and printing service, a web solutions web site. I would guess the list goes on. Is he a wheeler dealer?
5] eBay feedback is 99.1% positive. Very good. Too good to be true?
6] No mention in his sales talk that 'you' have to find some other person with an XP CD in order to install XP and then use the sticker on the box.
None of this, in itself, is proof positive that there is anything wrong with this seller. But all together, I would have walked away. I'm in the UK also, and I would not have touched that product.
As Bob said here;
if you have a relative/friend who you can borrow their XP CD off, then you can install XP from that and try the key from the sticker. That way you get to find out if the key is genuine or not.
But you still need the drivers for the hardware for that machine. Belarc Advisor will help you identify the hardware and any software once you have XP installed, then you hunt down the drivers.