Under the GDPR, personal data is that which identifies/links to you as a living individual, which includes your name, DOB, address/postcode, email, telephone number, memberships number etc
Sensitive personal data covers areas such as:
race
ethnic background
political opinions
religious beliefs
trade union membership
genetics
biometrics (where used for identification)
health
sex life or orientation
https://www.gov.uk/data-protection
Trade unions are listed, which can be particularly sensitive when considering employment, protest groups are not listed but are a fair call with regards to the potential links to beliefs / politics and sensitivities on differing opinions and the influence it could have on someone’s opinion of you in unrelated areas.
Membership of a group does not fall into the same level, unless there would be factors relating to the type of group.
Airsoft could be something you feel touchey about publicising it.
However, you are attempting to buy an Airsoft gun from someone which implies you play Airsoft and they are also legally liable if they sell to you without a valid purpose.
You have given them the elements of personal data, and there would be no GDPR breach in confirming status under the UKARA.
The entire purpose of the UKARA player database is to validate an individuals status, and the GDPR declaration states that information will be shared among parties - and they are not restricted to retailers only (it does state that they are the primary purpose). The specifics on the wording from the UKARA may vary over time and also with a site specific form as you sign up as a member of a site which is then added to the central UKARA player database.
Note that a seller does not get provided with the members details. The UKARA or a site confirm or deny that the records match