So, what it sounds like is that 6 of the sheep have slaughter tags in them. These are single yellow tags, which only display the flock number visually on the tags. They do have an individual number, but this is only stored on the electronic chip, so you would need a reader to find out what it is.
As they are this years' lambs, this is a perfectly legal/acceptable thing for the vendor to have done, but maybe not all that helpful - did you discuss with him what you want the sheep for? Of course, he may well have tagged them at birth not expecting to sell them for breeding. If you are wanting to fatten and slaughter them before 12 months, the single tag is completely fine, and you don't need to do anything. However, if you want to keep them beyond 12 months, you need to double tag them. You can remove the tags and re tag with a pair (could be yellow and purple to match the others, we use a different colour every year so we know instantly how old they are) and you MUST record that you have done this in your flock register.
Pairs of tags will display both the flock number and an individual number visually on the tag, as well as this being stored on the electronic chip. The two tags, yellow and purple in your case, will both display both numbers, i.e. they should look exactly the same except for the colour. If the flock number is different to the flock number of the vendor, then you are right, that would mean he had bought them in and sold them to you.
The vendor should have sent you a list of numbers. For your flock record, you could either record the two you have the full numbers for correctly and just record the other 6 as '6 ewe lambs ex flock UK WHATEVER, retag the six and then record the change, and I'd be reasonably confident if you had an inspection you should be OK as you've tried your best, or ideally find someone who can read the elctronic chips for you (not sure where you are, if you are in the Scottish Borders/North Northumberland I'll come and do it for you) - maybe the vendor can? and then record all the numbers properly.
Re their ears, how sore are they? If it is just a bit of gunk round the hole, I would clean it all off then spray with some antibiotic spray (both sides) and leave the tags for now, but if the ear is swollen, you'd need to check for infection tracks - it can burst right out of the tip of the ear so feel all over with your fingernails for scabs, then thoroughly clean. In general, you shouldn't be removing tags, but if it is a welfare issue, then do, but ensure you keep the old tags until you've recorded the replacement numbers.