If you continue to use mobile tethering and your plan is limited to 5GB, it should be way more than enough for using ebay/paypal.
I strongly suggest keeping an eye on perfmon program while you're tethering. This will help you gauge any rogue processes that consume your bandwidth.
You can find it quickly by typing perfmon.exe /res in command window or just go to START > ALL PROGRAMS > ACCESSORIES > SYSTEM TOOLS > RESOURCE MONITOR.
| 
