REST api: requesting multiple resources in a single get [duplicate]

Answer 1

I would recommend thinking of it like you are listing multiple representations of the resource filtered by id. As such you make a GET request to the base resource:

https://example.com/api/v1/products

And filter the response list by id:

https://example.com/api/v1/products?id=1,2,3

Answer 2

Your suggestion of ids separated with commas is good enough.

It would be instructive to examine some public REST APIs to see how they handle. For ex, the StackExchange API separates ids with a semi-colon - https://api.stackexchange.com/docs/answers-by-ids