Is a UserID unique across all eBay sites?

Yes, a UserID is unique across all eBay sites (Gateways) and if you find the same UserID on multiple sites, it refers to the same seller. To find out where the User is registered, you can make a call to GetUser.

Here is a sample request:
<?xml version="1.0" encoding="utf-8"?>
<GetUserRequest xmlns="urn:ebay:apis:eBLBaseComponents">
<Version>467</Version>
<UserID>userid</UserID>
<RequesterCredentials>
<eBayAuthToken>***</eBayAuthToken>
</RequesterCredentials>
</GetUserRequest>

You will get a response similar to this:
<?xml version="1.0" encoding="utf-8"?>
<GetUserResponse xmlns="urn:ebay:apis:eBLBaseComponents">
<Timestamp>2006-07-05T19:03:12.857Z</Timestamp>
<Ack>Success</Ack>
<Version>467</Version>
<Build>e467_core_Bundled_3133358_R1</Build>
<User>
<AboutMePage>false</AboutMePage>
<EIASToken>nY+sHZ2PrBmdj6wVnY+sEZ2PrA2dj6wJlYKlD5KDpA2dj6x9nY+seQ==</EIASToken>
<Email>Invalid Request</Email>
<FeedbackScore>1</FeedbackScore>
<UniqueNegativeFeedbackCount>0</UniqueNegativeFeedbackCount>
<UniquePositiveFeedbackCount>1</UniquePositiveFeedbackCount>
<PositiveFeedbackPercent>100.0</PositiveFeedbackPercent>
<FeedbackPrivate>false</FeedbackPrivate>
<FeedbackRatingStar>None</FeedbackRatingStar>
<IDVerified>false</IDVerified>
<eBayGoodStanding>true</eBayGoodStanding>
<NewUser>false</NewUser>
<RegistrationDate>2001-03-31T20:00:34.000Z</RegistrationDate>
<Site>US</Site>
<Status>Confirmed</Status>
<UserID>userid</UserID>
<UserIDChanged>false</UserIDChanged>
<UserIDLastChanged>2004-06-25T03:24:24.000Z</UserIDLastChanged>
<VATStatus>NoVATTax</VATStatus>
<SellerInfo>
<AllowPaymentEdit>false</AllowPaymentEdit>
<CheckoutEnabled>true</CheckoutEnabled>
<CIPBankAccountStored>false</CIPBankAccountStored>
<GoodStanding>true</GoodStanding>
<LiveAuctionAuthorized>false</LiveAuctionAuthorized>
<MerchandizingPref>OptIn</MerchandizingPref>
<QualifiesForB2BVAT>false</QualifiesForB2BVAT>
<SellerGuaranteeLevel>NotEligible</SellerGuaranteeLevel>
<SellerLevel>None</SellerLevel>
<SchedulingInfo>
<MaxScheduledMinutes>30240</MaxScheduledMinutes>
<MinScheduledMinutes>0</MinScheduledMinutes>
<MaxScheduledItems>3000</MaxScheduledItems>
</SchedulingInfo>
<StoreOwner>false</StoreOwner>
<ExpressEligible>false</ExpressEligible>
</SellerInfo>
<eBayWikiReadOnly>false</eBayWikiReadOnly>
</User>
</GetUserResponse>