I brought my Spirit-700 home, partially disassembled, in the trunk & backseat of my previous beater 2002 Buick LeSabre.
Having done that once - If I were going to attempt to do it again, I would suggest the following:
#1 - Only attempt this if you are at least a 4 on a scale of 1-10 for mechanical inclination (1="dunce" / 10=certified aircraft mechanic)
#2 - Pack a socket set, screwdrivers, and a sturdy pliers or two for disassembly duty.
#3 - Pack some flat cardboard to line the trunk and/or back seat if you do not have a liner. Grill gunk can be VERY DIFFICULT to remove from upholstry / carpeting
#4 - Take a few big garbage bags for the greasier stuff
#5 - pack some masking tape and a sharpie if you need to tag stuff when you take it apart. (as reminders, so that you can get it back together later)
Follow these steps to reduce the size / manage damage / get it to fit in manageable "chunks":
A: Remove loose stuff (grates, flavorizer bars. The lower metal drip-pan slides out like a drawer and usually contains a lot of gunk, etc. - put these in a box/garbage bag) If you can, toss the foil drip-catcher on-site, but be sure to keep the wire retainer and clip for it.
B. Remove the lid - this is usually easy. (2) clevis pins usually hold the end-caps of the hood to the body of the grill.
C. Detach the "legs" from the cart - keeping the upper part of the cart attached to the lower grill housing. The cart can usually fold-up and/or be disassembled into manageable-size sub-assemblies. The lower housing with upper cart & gas manifold/valves etc will usually squeeze into a decent-size vehicle.
If you go prepared, this should take about 20-minutes .
Lastly - be courteous. Be sure to arrange this with the seller beforehand, so that you don't keep them while you knock-down your new prize. And don't get grease/gunk on their property.