I am looking to get a new crankset for my bike and dont really have a ton of money. I came across this, https://cgi.ebay.com/Shimano-Ultegra-...35611651837967, I know it is last years model, but does anyone know about the quality of the crankset. Also could someone enlighten me on the different crank options like 39/53t vs 50/34 ect and what the benefit of having one size over another is

That's an excellent crank - I have a 50/34 on one of my bikes. Your question about sizes is puzzling to me - wouldn't you get the same gearing you already have? Long story short - be mindful of the gear ratios (bigger numbers = harder gears) and also the crank arm length - again, you want to match what already fits you. Finally, you'll need to make sure that your bottom bracket is compatible with the cranks. If you're switching out a Shimano 10sp crank, then you'll be fine with this one.

Edit:nvrmind, I see that the crank comes with a bottom bracket, just make sure it'll fit your frame.