In my opinion the price could be wide ranging depending on:
a) where you are selling it
b) who is buying it
A. Where you are selling it:
1. Public auction (i.e. Manheim, NOT online auctions)
You are looking at the lower end with this car selling anywhere between £500 and £850 as dealers simply won't pay more regardless of history (example is my BMW 325 which I spend £2250 on, and sold for £800! ouch!). If there are plenty of private punters there on the day, it could go up to £1000 at most.
2. Private advert in local paper
You will get more for the car here than option 1, as you can chat with the potential buyers and inform them of all the work that the engine has had done to it. I reckon you could get between £900-£1350 this way. Again though at the £900 mark it would sell like a hot cake whereas for £1350 you may have to hold out a little longer.
3. Private advert in large national car mag
Costs you more to place advert but you get more exposure and usually prices are more inflated. I reckon you could get up to £1500 here, although if you were a dealership they would put your car in at the £1750 mark, with exhaust botched.
4. Ebay
Probably the most variable selling place of all. Anywhere between £750-£2000 depending on what your advert is like, what the photos are like, who is bidding on the day, what date, day of the week and time the auction is finishing, whether 2 stock broker friends are bored so decide to outbid each other on your Omega for the fun of spending money (in which case it could go for £10,000!).
B. Who is buying it
1. Omega enthusiast
Will appreciate the value of the vehicle based on how much you have done to it. Will either be adding to an existing collection of Omega's or looking for a lucky car to pamper. However there are variants even within this category as some will be aware of values and won't want to spend much whereas others wont be bothered as much so this introduces some flexibility on how much they are prepared to pay.
2. Dealer
Won't give a toss what you've spent on it will simply look at its present condition (purely to work out its resale value) and will then offer you a much lower price that you will be insulted by. Then some haggling may start but most dealers are surprisingly inflexible with haggling as always in the back of their mind a voice is saying 'plenty more fish in the sea'.
3. Family man
Much more savvy than in the past but willing to pay the right price for the right car as wants a vehicle to drive his caravan down to Cornwall.
4. Boy Racer
Will call you to chat about the car. You'll spend an hour talking to him all about the top speed, torque, and optional mods, then he'll tell you he cant get insurance on it anyway!! Nah thats not always true I mustn't be unfair. I was driving a Porsche 924 turbo at 22. Insurance cost me £1000 a year but it was worth every penny! I was living with my parents with hardly any outgoings.
I hate putting people into pidgeon holes but that's what I've just done!!! To summarise, there are so many more variants to consider these days, and I know many dealers who respect Glasses guides calculations of value, but do not put any stock to that on the day. Good example is location. On average cars sell in London for up to 10% more than rest of UK. I reckon overall you are looking at the £1250 mark, but others may disagree with this and I won't defend this estimate at all.
Excuse my essay but I've been going to auctions since I was 7 and I think a single value is much to prescriptive!
