Can you please check my relationships and suggest me any improvements?

thanx

Any brief description of requirements will help in understanding this model.

By saying requirements what do you mean?
It's a system for Car sales management....
Basically the relationship I'm mostly not sure of is the one between the SaleItem and TBLservices & TBLCars...I want the user to enter the id of the item sold, but i want it to be taken from 2 different tables....like when someone buys a car, he'll enter the CarID in the same field he'd enter the serviceid if he bought a service...
I hope I managed to explain it

Yea, services and Car ... what if a customer wants to purchase both - he buys a car and purchases some service plan for the same car. Or already has a car and wants just to purchase some service. (Is it allowed?)
I suggest it should have two entities - both carid and serviceid - preferably optional.
Both are independent of each other.

I assume the payment table is duplicated in the diagram.

I don't wanna do that, because it will not look good when printing reports. I've been told to create an update query for that....

But how do you distinguish carid from serviceid ?
ok, you have different sequence/series for both ids. But every time you get the id, you'll have to search two different places to determine whether it's a carid or a serviceid.
How will this be done in reports ?
You can have a flag indicating the type of id.
like .. if flag = 'C' then the id belongs to carid
if 'S' then it's a serviceid.

Update query ? Dont' know ...

Why do you have two tblPayments? They look to be the same.