The reason we don't do this is because it adds to much complexity for the average person. Since 99% of people only have 1 eBay userId we'd be adding more fields and features that would only confuse those people and make it harder to add a snipe for the benefit of 1 in 100 people. And I doubt it's even that high. Probably closer to 1 in 200 or 300.
We'll keep it in mind and see about asking about it on our next poll perhaps. I could be wrong but I don't believe that many people have multiple Id's for buying. A good few people have one for buying and one for selling.