Not sure if it'd be out of your budget but we (and a couple of others on DDN) use Kashflow (
http://kashflow.aislinks.com - yes it's a referral link but it also gets you £1+VAT per month saving,
http://www.kashflow.co.uk to pay for the full price)
It is a business accounts software, based online so Mac/PC is no issue, nor is which computer you use (and there is an iPhone app if you have one). It would allow you to keep a note of items sold, purchased etc. You could also set up projects for your more project style things, you can keep notes and info in that and link it to purchases and invoices.
You can sign up and get the first 2 months for free so you get a good 60 days to give it a go and see if it's what you need before you start paying. Plus it will do invoices/quotes for you if you need them, and repeat automated billing if you need it now or in the future.
I'll also give you plenty of reports to help with telling the tax man about anything and you can easily categorise sales so that specific items/profit don't get included (eg. selling your personal possessions would not require to be declared to the tax man).
It's £15.99 + VAT per month or £14.99 + VAT with the referral link. Whilst it may sound like more cost, it is really worth the saved time that you'll hopefully find. As I say, not sure if it covers everything you need but it's worth a free look around to check
