Good record keeping helps when tax time comes around, but you do not need to report the reimbursements unless the MSC reports it and you need to write off that income.
Best part about the reimbursements (and why I focus on those) is that it's tax free income. If you focus on shops that reimburse for things you would otherwise purchase anyway, and have a FT job, the tax benefits may outweigh any shops fees collected!
Not that I know of. In general, the things to look out for are shops that offer a flat fee and require a purchase.
If a shop pays $10 and requires you to buy an item (say you purchase a $4 item), then your fee is $6 and the reimbursement is $4, but the MSC would report you making $10, provided that you earn over $600 from them in the tax year and your income gets reported at all. You would then need to claim and deduct all of the purchased items against that stated income. None-the-less, you should be keeping track of that stuff for your taxes and reporting the income only.
If you work mainly for companies that pay a fee and reimburse for expenses, it much easier to track, IMHO. Anything reported that's not an even dollar amount sets off my radar, since most shop fees are even amounts. MSCs are not supposed to include reimbursements in a 1099 but some do; usually out of misunderstandings or mis-categorizing the expense in my experience. If you do 100 shops for a MSC that pays $10 + reimbursement, you should get a 1099 for $1,000, claim that as your income and not worry about the reimbursements.