You don't need to register twice to do different self-employed jobs as you only have one UTR number (the registration number for self-employment). You can report it on separate pages of the tax return, though, if you wish. The income from all self-employment will be added up to establish whether you can apply for Class 2 NIC exemption. So if your husband makes £3k profit from job X and another £3k from job Y then he can't apply for the exemption as his total income from self-employment is £6k (over the exemption limit).
As mentioned before, he can also be employed at the same time, there's a separate page to complete on the SA return for employment income.
If your husband makes losses on one self-employment and profits on another, then losses CAN be set against profits, provided he was actually intending to make a profit from both of them (i.e. was running his business on commercial terms rather than "mates rates" or something).
You can work for your husband's business but if you take any income then you need to declare it and pay tax on it. Of course, you can do all the work for free (as I understand you already have your own income utilising your tax free allowances so there's no advantage in giving you an income through his business). The business can still be declared as your husband's since - let's be fair - how is HMRC ever going to know who actually puts the items on ebay etc...
If the ebay business takes off, though, be careful about VAT limits. If you sell more than £70k worth of items during any 12-month period then you need to register for VAT (unless you're selling some non-VATable items). THis isn't an immediate concern but worth bearing in mind if the business goes well.