I was reading on POAS.co.uk yesterday or the day before, that the test's have to be able to read at least what it sayd they can, so if you buy a multi pack of 2 tests and (for example) if the box says it can read 50miu it doesn't mean that every test in that box will read at 49miu, some can be very very varied, the example they gave were FR, in a lab test one of the FR test's were found to give a BFP at 4.5miu
Most women do always have a certain level of HCG in their system, any thing over 5 is considered to be pregnant, so if it was going off the cheap 10miu ebay's tests, then a feasable argument would be that if a FR can read HCG at such a low amount, then so could the 10miu ones, and that what they are picking up and showing as a BFP is in actual fact the HCG that occurs naturally in our bodies.
BUT, and it is a big BUT, I can clearly see two BFP's on CB tests on your profile, CB's IIRC are stated as 25miu? Which means you are either pregnant, or have a very very high 'natural' level of HCG for whatever reason, which I think is very un-common.
Is it worth going to your GP, asking for bloods?