...she gets paid...

While there is some weird stuff in her collection even before the series begin, some she gains during the series as payments for the wishes in the various moments of both series (xxxHolic and Tsubasa: Reservoir Chronicle ).
The storage room in the shop hides lot of interesting things. I’ll write two lists, which will be the first one of stuff that appears in other CLAMP mangas also, and the second that has connection just to the xxxHolic and Tsubasa: Reservoir Chronicle series.

I. High Moon Vase – it’s the same vase that Rikuou and Kazahaya retrieve in volume 2 of Gouhou drug.

II. Sakura’s first staff (replica) – the replica of the staff Sakura Kinamoto used to command Clow Cards (Card Captor Sakura).

III. Sakura’s Star staff (real) – it’s the staff Sakura used in the second part of the series to command Sakura Cards. This we find at the end of Tsubasa: Reservoir Chronicle , is the payment of Shaoran (real) mother, who is Sakura (clone) reborn, and was given to her in a dream by Sakura Kinamoto from Card Captor Sakura.

IV. Kerochan (peluche) – this is the copy of the Sun guardian small form (Card Captor Sakura), we never found out what is it used for.

I'm trying to find the image, stupidly I forgot to write down the volume and page

V. Earheadphones – during one of the first missions Watanuki uses them to communicate with Yuuko. The look exactly as Freya and Chii (Elda)’s ears from Chobits.

VII. Sakura Kinamoto (Card Captor Sakura)’s bag - In volume 6 Watanuki uses it while doing a job for Yuuko.

VII. Birdcage and birds – those look awfully as those in the manga Clover

VIII. Clow’s Glasses – they seem like his, they are in a box with his magical insignia. Later Watanuki wears them when his glasses get broken by a nasty spirit.

XI. Ashura and Yasha’s swords – she gets them as payment from Ashura that appears in Tsubasa: Reservoir Chronicle , but they look the same as the ones in RG Veda.

There is a rumor that she also owns and Hikaru Shido’s (Magic Knight Rayearth) sword, but I haven’t been able to notice them yet, I like searching for this stuff, so if I find them I shall put them in the list.

And now on to the second list that is about the stuff part of the two series.

1. Child seat – she gets it as a payment from the woman addicted with the computer and social networks (H – Vol 1).

2. Fai’s tattoo and staff – one was the payment to travel dimensions and never going back to his own world (T – Vol 1) and the other was payment for the help to enter the castle in the Secret Country (T – Vol 3).

3. Kurogane’s Ginryuu – it’s the payment to get home (T – Vol 1)

4. Watanuki’s memories – the payment before the series begun, it’s the payment for the group to be able to go to the Clow Country and find Fei Wang. (T – Vol 23)

5. Sakura (clone)’s memories of Shaoran (clone) – the payment to travel dimensions (T – Vol 1)

6. Sakura (clone), Shaoran (clone), Fai and Kurogane’s original clothes – they forgot the clothes in the country of Shara and so Yuuko took them, if they want them back, they have to pay a price. (T – Vol 11)

7. The tube with Shaoran (clone reborn) and Sakura (clone reborn) – this is the tube that goes open in the final battle, letting out the two to help the real two.

8. The tube with Sakura (clone)’s soul – the soul vanishes along with Sakura, but it’s the one where Yuuko manages to keep her safe (T – Vol 19)

9. Carpet with butterflies – every time the carpet is beaten, the butterflies fly away. (H – Vol 3)

10. The two Mokona Modoki – in the first Tsubasa: Reservoir Chronicle volume and the first and second of xxxHolic, they are woken and while the white goes with the Tsubasa: Reservoir Chronicle group, the black stays with Yuuko and Watanuki.

11. The baseball bat with the name Zantetsuhen (Goemon’s sword name from Lupin III) – she created it, to cut the computer of the addicted lady. (H – Vol 1)

12. Picture of Akari – the cat spirit that brings the houzuki. (H – Vol 6)

13. Twelve jars of pure water – she gets the location of the water thanks to the payment of Neko spirit for the hat, while Watanuki retrieves them (H – Vol 10), which are later used in Tsubasa: Reservoir Chronicle in the ruined Tokyo.

14. Monkey paw – found in Yuuko’s yard by the ambitious teacher, and returns back after it killed her. (H – Vol 3)