Here's a few examples of why my preferred buying method is individual items over collections. It takes time, but the returns are worth the effort.
The benefits are that you know exactly what you're getting, you have no remaindered collections to dispose of, and you can easily calculate selling prices and anticipated profit margins
1st image - PNG 1960 6d Postal Charges - bought for $10 from the US - sold for $36
2nd image - Saar 1921 5m - bought for $1.50 from the UK - sold for $10
3rd image - NZ 1893 2d Lighthouse with Z flaw - bought for $15 from Canada - sold for $50
4th image - Netherlands 1951 Seagulls - bought for $62 from Canada - sold for $160
Expenditure $88.50, sales value $256, profit after fees around $150